--- a/mercurial/cmdutil.py	Tue Jan 16 23:50:01 2018 +0900
+++ b/mercurial/cmdutil.py	Sun Oct 15 23:08:45 2017 +0530
@@ -42,6 +42,7 @@
     revlog,
     revset,
     revsetlang,
+    rewriteutil,
     scmutil,
     smartset,
     templatekw,
@@ -713,6 +714,92 @@
 
     raise error.UnknownCommand(cmd, allcmds)
 
+def changebranch(ui, repo, revs, label):
+    """ Change the branch name of given revs to label """
+
+    with repo.wlock(), repo.lock(), repo.transaction('branches'):
+        # abort in case of uncommitted merge or dirty wdir
+        bailifchanged(repo)
+        revs = scmutil.revrange(repo, revs)
+        if not revs:
+            raise error.Abort("empty revision set")
+        roots = repo.revs('roots(%ld)', revs)
+        if len(roots) > 1:
+            raise error.Abort(_("cannot change branch of non-linear revisions"))
+        rewriteutil.precheck(repo, revs, 'change branch of')
+        if repo.revs('merge() and %ld', revs):
+            raise error.Abort(_("cannot change branch of a merge commit"))
+        if repo.revs('obsolete() and %ld', revs):
+            raise error.Abort(_("cannot change branch of a obsolete changeset"))
+
+        # make sure only topological heads
+        if repo.revs('heads(%ld) - head()', revs):
+            raise error.Abort(_("cannot change branch in middle of a stack"))
+
+        replacements = {}
+        # avoid import cycle mercurial.cmdutil -> mercurial.context ->
+        # mercurial.subrepo -> mercurial.cmdutil
+        from . import context
+        for rev in revs:
+            ctx = repo[rev]
+            oldbranch = ctx.branch()
+            # check if ctx has same branch
+            if oldbranch == label:
+                continue
+
+            def filectxfn(repo, newctx, path):
+                try:
+                    return ctx[path]
+                except error.ManifestLookupError:
+                    return None
+
+            ui.debug("changing branch of '%s' from '%s' to '%s'\n"
+                     % (hex(ctx.node()), oldbranch, label))
+            extra = ctx.extra()
+            extra['branch_change'] = hex(ctx.node())
+            # While changing branch of set of linear commits, make sure that
+            # we base our commits on new parent rather than old parent which
+            # was obsoleted while changing the branch
+            p1 = ctx.p1().node()
+            p2 = ctx.p2().node()
+            if p1 in replacements:
+                p1 = replacements[p1][0]
+            if p2 in replacements:
+                p2 = replacements[p2][0]
+
+            mc = context.memctx(repo, (p1, p2),
+                                ctx.description(),
+                                ctx.files(),
+                                filectxfn,
+                                user=ctx.user(),
+                                date=ctx.date(),
+                                extra=extra,
+                                branch=label)
+
+            commitphase = ctx.phase()
+            overrides = {('phases', 'new-commit'): commitphase}
+            with repo.ui.configoverride(overrides, 'branch-change'):
+                newnode = repo.commitctx(mc)
+
+            replacements[ctx.node()] = (newnode,)
+            ui.debug('new node id is %s\n' % hex(newnode))
+
+        # create obsmarkers and move bookmarks
+        scmutil.cleanupnodes(repo, replacements, 'branch-change')
+
+        # move the working copy too
+        wctx = repo[None]
+        # in-progress merge is a bit too complex for now.
+        if len(wctx.parents()) == 1:
+            newid = replacements.get(wctx.p1().node())
+            if newid is not None:
+                # avoid import cycle mercurial.cmdutil -> mercurial.hg ->
+                # mercurial.cmdutil
+                from . import hg
+                hg.update(repo, newid[0], quietempty=True)
+
+        ui.status(_("changed branch on %d changesets\n") % len(replacements))
+
 def findrepo(p):
     while not os.path.isdir(os.path.join(p, ".hg")):
         oldp, p = p, os.path.dirname(p)

--- a/mercurial/commands.py	Tue Jan 16 23:50:01 2018 +0900
+++ b/mercurial/commands.py	Sun Oct 15 23:08:45 2017 +0530
@@ -1002,7 +1002,9 @@
 @command('branch',
     [('f', 'force', None,
      _('set branch name even if it shadows an existing branch')),
-    ('C', 'clean', None, _('reset branch name to parent branch name'))],
+     ('C', 'clean', None, _('reset branch name to parent branch name')),
+     ('r', 'rev', [], _('change branches of the given revs (EXPERIMENTAL)')),
+    ],
     _('[-fC] [NAME]'))
 def branch(ui, repo, label=None, **opts):
     """set or show the current branch name
@@ -1034,10 +1036,13 @@
     Returns 0 on success.
     """
     opts = pycompat.byteskwargs(opts)
+    revs = opts.get('rev')
     if label:
         label = label.strip()
 
     if not opts.get('clean') and not label:
+        if revs:
+            raise error.Abort(_("no branch name specified for the revisions"))
         ui.write("%s\n" % repo.dirstate.branch())
         return
 
@@ -1047,13 +1052,23 @@
             repo.dirstate.setbranch(label)
             ui.status(_('reset working directory to branch %s\n') % label)
         elif label:
+
+            scmutil.checknewlabel(repo, label, 'branch')
+            if revs:
+                # XXX: we should allow setting name to existing branch if the
+                # branch of root of the revs is same as the new branch name
+                if label in repo.branchmap():
+                    raise error.Abort(_('a branch of the same'
+                                        ' name already exists'))
+                return cmdutil.changebranch(ui, repo, revs, label)
+
             if not opts.get('force') and label in repo.branchmap():
                 if label not in [p.branch() for p in repo[None].parents()]:
                     raise error.Abort(_('a branch of the same name already'
                                        ' exists'),
                                      # i18n: "it" refers to an existing branch
                                      hint=_("use 'hg update' to switch to it"))
-            scmutil.checknewlabel(repo, label, 'branch')
+
             repo.dirstate.setbranch(label)
             ui.status(_('marked working directory as branch %s\n') % label)
 

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-branch-change.t	Sun Oct 15 23:08:45 2017 +0530
@@ -0,0 +1,296 @@
+Testing changing branch on commits
+==================================
+
+Setup
+
+  $ cat >> $HGRCPATH << EOF
+  > [alias]
+  > glog = log -G -T "{rev}:{node|short} {desc}\n{branch} ({bookmarks})"
+  > [experimental]
+  > evolution = createmarkers
+  > [extensions]
+  > rebase=
+  > EOF
+
+  $ hg init repo
+  $ cd repo
+  $ for ch in a b c d e; do echo foo >> $ch; hg ci -Aqm "Added "$ch; done
+  $ hg glog
+  @  4:aa98ab95a928 Added e
+  |  default ()
+  o  3:62615734edd5 Added d
+  |  default ()
+  o  2:28ad74487de9 Added c
+  |  default ()
+  o  1:29becc82797a Added b
+  |  default ()
+  o  0:18d04c59bb5d Added a
+     default ()
+
+  $ hg branches
+  default                        4:aa98ab95a928
+
+Try without passing a new branch name
+
+  $ hg branch -r .
+  abort: no branch name specified for the revisions
+  [255]
+
+Setting an invalid branch name
+
+  $ hg branch -r . a:b
+  abort: ':' cannot be used in a name
+  [255]
+  $ hg branch -r . tip
+  abort: the name 'tip' is reserved
+  [255]
+  $ hg branch -r . 1234
+  abort: cannot use an integer as a name
+  [255]
+
+Change on non-linear set of commits
+
+  $ hg branch -r 2 -r 4 foo
+  abort: cannot change branch of non-linear revisions
+  [255]
+
+Change in middle of the stack (linear commits)
+
+  $ hg branch -r 1::3 foo
+  abort: cannot change branch of changeset with children
+  [255]
+
+Change with dirty working directory
+
+  $ echo bar > a
+  $ hg branch -r . foo
+  abort: uncommitted changes
+  [255]
+
+  $ hg revert --all
+  reverting a
+
+Change on empty revision set
+
+  $ hg branch -r 'draft() - all()' foo
+  abort: empty revision set
+  [255]
+
+Changing branch on linear set of commits from head
+
+Without obsmarkers
+
+  $ hg branch -r 3:4 foo --config experimental.evolution=!
+  changed branch on 2 changesets
+  saved backup bundle to $TESTTMP/repo/.hg/strip-backup/62615734edd5-e86bd13a-branch-change.hg (glob)
+  $ hg glog
+  @  4:3938acfb5c0f Added e
+  |  foo ()
+  o  3:9435da006bdc Added d
+  |  foo ()
+  o  2:28ad74487de9 Added c
+  |  default ()
+  o  1:29becc82797a Added b
+  |  default ()
+  o  0:18d04c59bb5d Added a
+     default ()
+
+  $ hg branches
+  foo                            4:3938acfb5c0f
+  default                        2:28ad74487de9 (inactive)
+
+With obsmarkers
+
+  $ hg branch -r 3::4 bar
+  changed branch on 2 changesets
+  $ hg glog
+  @  6:7c1991464886 Added e
+  |  bar ()
+  o  5:1ea05e93925f Added d
+  |  bar ()
+  o  2:28ad74487de9 Added c
+  |  default ()
+  o  1:29becc82797a Added b
+  |  default ()
+  o  0:18d04c59bb5d Added a
+     default ()
+
+  $ hg branches
+  bar                            6:7c1991464886
+  default                        2:28ad74487de9 (inactive)
+
+Change branch name to an existing branch
+
+  $ hg branch -r . default
+  abort: a branch of the same name already exists
+  [255]
+
+Changing on a branch head which is not topological head
+
+  $ hg branch -r 2 stable
+  abort: cannot change branch of changeset with children
+  [255]
+
+Enabling the allowunstable config and trying to change branch on a branch head
+which is not a topological head
+
+  $ echo "[experimental]" >> .hg/hgrc
+  $ echo "evolution.allowunstable=yes" >> .hg/hgrc
+  $ hg branch -r 2 foo
+  changed branch on 1 changesets
+  2 new orphan changesets
+
+Changing branch of an obsoleted changeset
+
+  $ hg branch -r 4 foobar
+  abort: hidden revision '4' was rewritten as: 7c1991464886!
+  (use --hidden to access hidden revisions)
+  [255]
+
+  $ hg branch -r 4 --hidden foobar
+  abort: cannot change branch of a obsolete changeset
+  [255]
+
+Make sure bookmark movement is correct
+
+  $ hg bookmark b1
+  $ hg glog -r '.^::'
+  @  6:7c1991464886 Added e
+  |  bar (b1)
+  *  5:1ea05e93925f Added d
+  |  bar ()
+  ~
+
+  $ hg branch -r '(.^)::' wat --debug
+  changing branch of '1ea05e93925f806d875a2163f9b76764be644636' from 'bar' to 'wat'
+  committing files:
+  d
+  committing manifest
+  committing changelog
+  new node id is 343660ccab7400da637bd6a211d07f413536d718
+  changing branch of '7c19914648869f5b02fc7fed31ddee9783fdd680' from 'bar' to 'wat'
+  committing files:
+  e
+  committing manifest
+  committing changelog
+  new node id is de1404b45a69f8cc6437d7679033ee33e9efb4ba
+  moving bookmarks ['b1'] from 7c19914648869f5b02fc7fed31ddee9783fdd680 to de1404b45a69f8cc6437d7679033ee33e9efb4ba
+  resolving manifests
+   branchmerge: False, force: False, partial: False
+   ancestor: 7c1991464886, local: 7c1991464886+, remote: de1404b45a69
+  changed branch on 2 changesets
+  updating the branch cache
+  invalid branchheads cache (served): tip differs
+
+  $ hg glog -r '(.^)::'
+  @  9:de1404b45a69 Added e
+  |  wat (b1)
+  *  8:343660ccab74 Added d
+  |  wat ()
+  ~
+
+Make sure phase handling is correct
+
+  $ echo foo >> bar
+  $ hg ci -Aqm "added bar" --secret
+  1 new orphan changesets
+  $ hg glog -r .
+  @  10:8ad1294c1660 added bar
+  |  wat (b1)
+  ~
+  $ hg branch -r . secret
+  changed branch on 1 changesets
+  $ hg phase -r .
+  11: secret
+
+  $ hg branches
+  secret                        11:38a9b2d53f98
+  foo                            7:8a4729a5e2b8
+  wat                            9:de1404b45a69 (inactive)
+  default                        2:28ad74487de9 (inactive)
+  $ hg branch
+  secret
+
+Changing branch of another head, different from one on which we are
+
+  $ hg glog
+  @  11:38a9b2d53f98 added bar
+  |  secret (b1)
+  *  9:de1404b45a69 Added e
+  |  wat ()
+  *  8:343660ccab74 Added d
+  |  wat ()
+  | o  7:8a4729a5e2b8 Added c
+  | |  foo ()
+  x |  2:28ad74487de9 Added c
+  |/   default ()
+  o  1:29becc82797a Added b
+  |  default ()
+  o  0:18d04c59bb5d Added a
+     default ()
+
+  $ hg branch
+  secret
+
+  $ hg branch -r 7 foobar
+  changed branch on 1 changesets
+
+The current branch must be preserved
+  $ hg branch
+  secret
+
+Changing branch on multiple heads at once
+
+  $ hg rebase -s 8 -d 12 --keepbranches -q
+
+  $ hg rebase -s 14 -d 1 --keepbranches -q
+
+  $ hg branch -r 0: stable
+  changed branch on 6 changesets
+  $ hg glog
+  @  23:6a5ddbcfb870 added bar
+  |  stable (b1)
+  o  22:baedc6e98a67 Added e
+  |  stable ()
+  | o  21:99ac7bf8aad1 Added d
+  | |  stable ()
+  | o  20:0ecb4d39c4bd Added c
+  |/   stable ()
+  o  19:fd45b986b109 Added b
+  |  stable ()
+  o  18:204d2769eca2 Added a
+     stable ()
+
+  $ hg branches
+  stable                        23:6a5ddbcfb870
+
+  $ hg branch
+  stable
+
+Changing to same branch name does not work
+
+  $ hg branch -r 19::21 stable
+  abort: a branch of the same name already exists
+  [255]
+
+Testing on merge
+
+  $ hg merge -r 20
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+
+  $ hg branch -r . abcd
+  abort: outstanding uncommitted merge
+  [255]
+  $ hg ci -m "Merge commit"
+  $ hg branch -r '(.^)::' def
+  abort: cannot change branch of a merge commit
+  [255]
+
+Changing branch on public changeset
+
+  $ hg phase -r 21 -p
+  $ hg branch -r 21 def
+  abort: cannot change branch of public changesets
+  (see 'hg help phases' for details)
+  [255]

--- a/tests/test-completion.t	Tue Jan 16 23:50:01 2018 +0900
+++ b/tests/test-completion.t	Sun Oct 15 23:08:45 2017 +0530
@@ -242,7 +242,7 @@
   backout: merge, commit, no-commit, parent, rev, edit, tool, include, exclude, message, logfile, date, user
   bisect: reset, good, bad, skip, extend, command, noupdate
   bookmarks: force, rev, delete, rename, inactive, template
-  branch: force, clean
+  branch: force, clean, rev
   branches: active, closed, template
   bundle: force, rev, branch, base, all, type, ssh, remotecmd, insecure
   cat: output, rev, decode, include, exclude, template