--- a/hgext/uncommit.py	Tue Nov 28 13:27:43 2017 -0500
+++ b/hgext/uncommit.py	Sun Sep 24 00:56:52 2017 +0530
@@ -29,6 +29,7 @@
     error,
     node,
     obsolete,
+    obsutil,
     pycompat,
     registrar,
     scmutil,
@@ -194,3 +195,124 @@
             with repo.dirstate.parentchange():
                 repo.dirstate.setparents(newid, node.nullid)
                 _uncommitdirstate(repo, old, match)
+
+def predecessormarkers(ctx):
+    """yields the obsolete markers marking the given changeset as a successor"""
+    for data in ctx.repo().obsstore.predecessors.get(ctx.node(), ()):
+        yield obsutil.marker(ctx.repo(), data)
+
+def _unamenddirstate(repo, predctx, curctx):
+    """"""
+
+    s = repo.status(predctx, curctx)
+    ds = repo.dirstate
+    copies = dict(ds.copies())
+    for f in s.modified:
+        if ds[f] == 'r':
+            # modified + removed -> removed
+            continue
+        ds.normallookup(f)
+
+    for f in s.added:
+        if ds[f] == 'r':
+            # added + removed -> unknown
+            ds.drop(f)
+        elif ds[f] != 'a':
+            ds.add(f)
+
+    for f in s.removed:
+        if ds[f] == 'a':
+            # removed + added -> normal
+            ds.normallookup(f)
+        elif ds[f] != 'r':
+            ds.remove(f)
+
+    # Merge old parent and old working dir copies
+    oldcopies = {}
+    for f in (s.modified + s.added):
+        src = curctx[f].renamed()
+        if src:
+            oldcopies[f] = src[0]
+    oldcopies.update(copies)
+    copies = dict((dst, oldcopies.get(src, src))
+                  for dst, src in oldcopies.iteritems())
+    # Adjust the dirstate copies
+    for dst, src in copies.iteritems():
+        if (src not in predctx or dst in predctx or ds[dst] != 'a'):
+            src = None
+        ds.copy(src, dst)
+
+@command('^unamend', [])
+def unamend(ui, repo, **opts):
+    """
+    undo the most recent amend operation on a current changeset
+
+    This command will roll back to the previous version of a changeset,
+    leaving working directory in state in which it was before running
+    `hg amend` (e.g. files modified as part of an amend will be
+    marked as modified `hg status`)
+    """
+
+    unfi = repo.unfiltered()
+
+    # identify the commit from which to unamend
+    curctx = repo['.']
+
+    with repo.wlock(), repo.lock(), repo.transaction('unamend'):
+        if not curctx.mutable():
+            raise error.Abort(_('cannot unamend public changesets'))
+
+        # identify the commit to which to unamend
+        markers = list(predecessormarkers(curctx))
+        if len(markers) != 1:
+            e = _("changeset must have one predecessor, found %i predecessors")
+            raise error.Abort(e % len(markers))
+
+        prednode = markers[0].prednode()
+        predctx = unfi[prednode]
+
+        if curctx.children():
+            raise error.Abort(_("cannot unamend a changeset with children"))
+
+        # add an extra so that we get a new hash
+        # note: allowing unamend to undo an unamend is an intentional feature
+        extras = predctx.extra()
+        extras['unamend_source'] = curctx.node()
+
+        def filectxfn(repo, ctx_, path):
+            try:
+                return predctx.filectx(path)
+            except KeyError:
+                return None
+
+        # Make a new commit same as predctx
+        newctx = context.memctx(repo,
+                                parents=(predctx.p1(), predctx.p2()),
+                                text=predctx.description(),
+                                files=predctx.files(),
+                                filectxfn=filectxfn,
+                                user=predctx.user(),
+                                date=predctx.date(),
+                                extra=extras)
+        # phase handling
+        commitphase = curctx.phase()
+        overrides = {('phases', 'new-commit'): commitphase}
+        with repo.ui.configoverride(overrides, 'uncommit'):
+            newprednode = repo.commitctx(newctx)
+
+        newpredctx = repo[newprednode]
+
+        changedfiles = []
+        wctx = repo[None]
+        wm = wctx.manifest()
+        cm = newpredctx.manifest()
+        dirstate = repo.dirstate
+        diff = cm.diff(wm)
+        changedfiles.extend(diff.iterkeys())
+
+        with dirstate.parentchange():
+            dirstate.setparents(newprednode, node.nullid)
+            _unamenddirstate(repo, newpredctx, curctx)
+
+        mapping = {curctx.node(): (newprednode,)}
+        scmutil.cleanupnodes(repo, mapping, 'unamend')

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-unamend.t	Sun Sep 24 00:56:52 2017 +0530
@@ -0,0 +1,366 @@
+Test for command `hg unamend` which lives in uncommit extension
+===============================================================
+
+  $ cat >> $HGRCPATH << EOF
+  > [alias]
+  > glog = log -G -T '{rev}:{node|short}  {desc}'
+  > [experimental]
+  > evolution = createmarkers, allowunstable
+  > [extensions]
+  > rebase =
+  > amend =
+  > uncommit =
+  > EOF
+
+Repo Setup
+
+  $ hg init repo
+  $ cd repo
+  $ for ch in {a..h}; do touch $ch; echo "foo" >> $ch; hg ci -Aqm "Added "$ch; done
+
+  $ hg glog
+  @  7:ec2426147f0e  Added h
+  |
+  o  6:87d6d6676308  Added g
+  |
+  o  5:825660c69f0c  Added f
+  |
+  o  4:aa98ab95a928  Added e
+  |
+  o  3:62615734edd5  Added d
+  |
+  o  2:28ad74487de9  Added c
+  |
+  o  1:29becc82797a  Added b
+  |
+  o  0:18d04c59bb5d  Added a
+  
+Trying to unamend when there was no amend done
+
+  $ hg unamend
+  abort: changeset must have one predecessor, found 0 predecessors
+  [255]
+
+Unamend on clean wdir and tip
+
+  $ echo "bar" >> h
+  $ hg amend
+
+  $ hg exp
+  # HG changeset patch
+  # User test
+  # Date 0 0
+  #      Thu Jan 01 00:00:00 1970 +0000
+  # Node ID c9fa1a715c1b7661c0fafb362a9f30bd75878d7d
+  # Parent  87d6d66763085b629e6d7ed56778c79827273022
+  Added h
+  
+  diff -r 87d6d6676308 -r c9fa1a715c1b h
+  --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/h	Thu Jan 01 00:00:00 1970 +0000
+  @@ -0,0 +1,2 @@
+  +foo
+  +bar
+
+  $ hg glog --hidden
+  @  8:c9fa1a715c1b  Added h
+  |
+  | x  7:ec2426147f0e  Added h
+  |/
+  o  6:87d6d6676308  Added g
+  |
+  o  5:825660c69f0c  Added f
+  |
+  o  4:aa98ab95a928  Added e
+  |
+  o  3:62615734edd5  Added d
+  |
+  o  2:28ad74487de9  Added c
+  |
+  o  1:29becc82797a  Added b
+  |
+  o  0:18d04c59bb5d  Added a
+  
+  $ hg unamend
+  $ hg glog --hidden
+  @  9:8da14a1fd653  Added h
+  |
+  | x  8:c9fa1a715c1b  Added h
+  |/
+  | x  7:ec2426147f0e  Added h
+  |/
+  o  6:87d6d6676308  Added g
+  |
+  o  5:825660c69f0c  Added f
+  |
+  o  4:aa98ab95a928  Added e
+  |
+  o  3:62615734edd5  Added d
+  |
+  o  2:28ad74487de9  Added c
+  |
+  o  1:29becc82797a  Added b
+  |
+  o  0:18d04c59bb5d  Added a
+  
+  $ hg diff
+  diff -r 8da14a1fd653 h
+  --- a/h	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/h	Thu Jan 01 00:00:00 1970 +0000
+  @@ -1,1 +1,2 @@
+   foo
+  +bar
+
+  $ hg exp
+  # HG changeset patch
+  # User test
+  # Date 0 0
+  #      Thu Jan 01 00:00:00 1970 +0000
+  # Node ID 8da14a1fd653c3f07fdad5760511c9e12652a306
+  # Parent  87d6d66763085b629e6d7ed56778c79827273022
+  Added h
+  
+  diff -r 87d6d6676308 -r 8da14a1fd653 h
+  --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/h	Thu Jan 01 00:00:00 1970 +0000
+  @@ -0,0 +1,1 @@
+  +foo
+
+  $ hg status
+  M h
+
+  $ hg log -r . -T '{extras % "{extra}\n"}' --config alias.log=log
+  branch=default
+  unamend_source=\xc9\xfa\x1aq\\\x1bva\xc0\xfa\xfb6*\x9f0\xbdu\x87\x8d}
+
+Using unamend to undo an unamed (intentional)
+
+  $ hg unamend
+  $ hg exp
+  # HG changeset patch
+  # User test
+  # Date 0 0
+  #      Thu Jan 01 00:00:00 1970 +0000
+  # Node ID 1c09887216571a9755fc9d2e7f0e41c2b49e341b
+  # Parent  87d6d66763085b629e6d7ed56778c79827273022
+  Added h
+  
+  diff -r 87d6d6676308 -r 1c0988721657 h
+  --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/h	Thu Jan 01 00:00:00 1970 +0000
+  @@ -0,0 +1,2 @@
+  +foo
+  +bar
+  $ hg diff
+
+Unamend on a dirty working directory
+
+  $ echo "bar" >> a
+  $ hg amend
+  $ echo "foobar" >> a
+  $ echo "bar" >> b
+  $ hg status
+  M a
+  M b
+
+  $ hg unamend
+
+  $ hg status
+  M a
+  M b
+
+  $ hg diff
+  diff -r 956548224719 a
+  --- a/a	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/a	Thu Jan 01 00:00:00 1970 +0000
+  @@ -1,1 +1,3 @@
+   foo
+  +bar
+  +foobar
+  diff -r 956548224719 b
+  --- a/b	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/b	Thu Jan 01 00:00:00 1970 +0000
+  @@ -1,1 +1,2 @@
+   foo
+  +bar
+
+Unamending an added file
+
+  $ hg ci -m "Added things to a and b"
+  $ echo foo > bar
+  $ hg add bar
+  $ hg amend
+
+  $ hg unamend
+  $ hg status
+  A bar
+
+  $ hg revert --all
+  forgetting bar
+
+Unamending a removed file
+
+  $ hg remove a
+  $ hg amend
+
+  $ hg unamend
+  $ hg status
+  R a
+  ? bar
+
+  $ hg revert --all
+  undeleting a
+
+Unamending an added file with dirty wdir status
+
+  $ hg add bar
+  $ hg amend
+  $ echo bar >> bar
+  $ hg status
+  M bar
+
+  $ hg unamend
+  $ hg status
+  A bar
+  $ hg diff
+  diff -r ff917aa01c86 bar
+  --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/bar	Thu Jan 01 00:00:00 1970 +0000
+  @@ -0,0 +1,2 @@
+  +foo
+  +bar
+
+  $ hg revert --all
+  forgetting bar
+
+Unamending in middle of a stack
+
+  $ hg glog
+  @  19:ff917aa01c86  Added things to a and b
+  |
+  o  12:956548224719  Added h
+  |
+  o  6:87d6d6676308  Added g
+  |
+  o  5:825660c69f0c  Added f
+  |
+  o  4:aa98ab95a928  Added e
+  |
+  o  3:62615734edd5  Added d
+  |
+  o  2:28ad74487de9  Added c
+  |
+  o  1:29becc82797a  Added b
+  |
+  o  0:18d04c59bb5d  Added a
+  
+  $ hg up 5
+  2 files updated, 0 files merged, 2 files removed, 0 files unresolved
+  $ echo bar >> f
+  $ hg amend
+  $ hg rebase -s 6 -d . -q
+
+  $ hg glog
+  o  23:2b41b42e192a  Added things to a and b
+  |
+  o  22:1860df151f01  Added h
+  |
+  o  21:49635b68477e  Added g
+  |
+  @  20:93f0e8ffab32  Added f
+  |
+  o  4:aa98ab95a928  Added e
+  |
+  o  3:62615734edd5  Added d
+  |
+  o  2:28ad74487de9  Added c
+  |
+  o  1:29becc82797a  Added b
+  |
+  o  0:18d04c59bb5d  Added a
+  
+
+  $ hg unamend
+  abort: cannot unamend a changeset with children
+  [255]
+
+Trying to unamend a public changeset
+
+  $ hg up
+  4 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg phase -r . -p
+  $ hg unamend
+  abort: cannot unamend public changesets
+  [255]
+
+Testing whether unamend retains copies or not
+
+  $ hg status
+  ? bar
+
+  $ hg mv a foo
+
+  $ hg ci -m "Moved a to foo"
+  $ hg exp --git
+  # HG changeset patch
+  # User test
+  # Date 0 0
+  #      Thu Jan 01 00:00:00 1970 +0000
+  # Node ID b087c66cada118d6c5487d3d7cb29cac239bd98a
+  # Parent  2b41b42e192adc425b106643c3fc89170d6b8add
+  Moved a to foo
+  
+  diff --git a/a b/foo
+  rename from a
+  rename to foo
+
+  $ hg mv b foobar
+  $ hg diff --git
+  diff --git a/b b/foobar
+  rename from b
+  rename to foobar
+  $ hg amend
+
+  $ hg exp --git
+  # HG changeset patch
+  # User test
+  # Date 0 0
+  #      Thu Jan 01 00:00:00 1970 +0000
+  # Node ID 9fa06fb09a83a86ec7368d15004dbc52ac1a5d2e
+  # Parent  2b41b42e192adc425b106643c3fc89170d6b8add
+  Moved a to foo
+  
+  diff --git a/a b/foo
+  rename from a
+  rename to foo
+  diff --git a/b b/foobar
+  rename from b
+  rename to foobar
+
+  $ hg mv c wat
+  $ hg unamend
+
+Retained copies in new prdecessor commit
+
+  $ hg exp --git
+  # HG changeset patch
+  # User test
+  # Date 0 0
+  #      Thu Jan 01 00:00:00 1970 +0000
+  # Node ID 4cf9987c9b941f615930e7c46307b4dbf35697cf
+  # Parent  2b41b42e192adc425b106643c3fc89170d6b8add
+  Moved a to foo
+  
+  diff --git a/a b/foo
+  rename from a
+  rename to foo
+
+Retained copies in working directoy
+
+  $ hg diff --git
+  diff --git a/b b/foobar
+  rename from b
+  rename to foobar
+  diff --git a/c b/wat
+  rename from c
+  rename to wat