--- a/mercurial/commands.py	Mon Feb 01 12:55:31 2021 +0100
+++ b/mercurial/commands.py	Thu May 07 16:50:26 2020 -0400
@@ -29,6 +29,7 @@
     bundlecaches,
     changegroup,
     cmdutil,
+    context as contextmod,
     copies,
     debugcommands as debugcommandsmod,
     destutil,
@@ -2464,6 +2465,16 @@
         (b'', b'from', b'', _(b'revision to diff from'), _(b'REV1')),
         (b'', b'to', b'', _(b'revision to diff to'), _(b'REV2')),
         (b'c', b'change', b'', _(b'change made by revision'), _(b'REV')),
+        (
+            b'',
+            b'merge',
+            False,
+            _(
+                b'show difference between auto-merge and committed '
+                b'merge for merge commits (EXPERIMENTAL)'
+            ),
+            _(b'REV'),
+        ),
     ]
     + diffopts
     + diffopts2
@@ -2544,13 +2555,31 @@
     to_rev = opts.get(b'to')
     stat = opts.get(b'stat')
     reverse = opts.get(b'reverse')
+    diffmerge = opts.get(b'merge')
 
     cmdutil.check_incompatible_arguments(opts, b'from', [b'rev', b'change'])
     cmdutil.check_incompatible_arguments(opts, b'to', [b'rev', b'change'])
     if change:
         repo = scmutil.unhidehashlikerevs(repo, [change], b'nowarn')
         ctx2 = scmutil.revsingle(repo, change, None)
-        ctx1 = ctx2.p1()
+        if diffmerge and ctx2.p2().node() != nullid:
+            pctx1 = ctx2.p1()
+            pctx2 = ctx2.p2()
+            wctx = contextmod.overlayworkingctx(repo)
+            wctx.setbase(pctx1)
+            with ui.configoverride(
+                {
+                    (
+                        b'ui',
+                        b'forcemerge',
+                    ): b'internal:merge3-lie-about-conflicts',
+                },
+                b'diff --merge',
+            ):
+                mergemod.merge(pctx2, wc=wctx)
+            ctx1 = wctx
+        else:
+            ctx1 = ctx2.p1()
     elif from_rev or to_rev:
         repo = scmutil.unhidehashlikerevs(
             repo, [from_rev] + [to_rev], b'nowarn'

--- a/tests/test-completion.t	Mon Feb 01 12:55:31 2021 +0100
+++ b/tests/test-completion.t	Thu May 07 16:50:26 2020 -0400
@@ -336,7 +336,7 @@
   debugwhyunstable: 
   debugwireargs: three, four, five, ssh, remotecmd, insecure
   debugwireproto: localssh, peer, noreadstderr, nologhandshake, ssh, remotecmd, insecure
-  diff: rev, from, to, change, text, git, binary, nodates, noprefix, show-function, reverse, ignore-all-space, ignore-space-change, ignore-blank-lines, ignore-space-at-eol, unified, stat, root, include, exclude, subrepos
+  diff: rev, from, to, change, merge, text, git, binary, nodates, noprefix, show-function, reverse, ignore-all-space, ignore-space-change, ignore-blank-lines, ignore-space-at-eol, unified, stat, root, include, exclude, subrepos
   export: bookmark, output, switch-parent, rev, text, git, binary, nodates, template
   files: rev, print0, include, exclude, template, subrepos
   forget: interactive, include, exclude, dry-run

--- a/tests/test-diff-change.t	Mon Feb 01 12:55:31 2021 +0100
+++ b/tests/test-diff-change.t	Thu May 07 16:50:26 2020 -0400
@@ -194,4 +194,108 @@
    9
    10
 
+merge diff should show only manual edits to a merge:
+
+  $ hg diff --merge -c 6
+  merging file.txt
+(no diff output is expected here)
+
+Construct an "evil merge" that does something other than just the merge.
+
+  $ hg co ".^"
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg merge -r 5
+  merging file.txt
+  0 files updated, 1 files merged, 0 files removed, 0 files unresolved
+  (branch merge, don't forget to commit)
+  $ echo 11 >> file.txt
+  $ hg ci -m 'merge 8 to y with manual edit of 11' # 7
+  created new head
+  $ hg diff -c 7
+  diff -r 273b50f17c6d -r 8ad85e839ba7 file.txt
+  --- a/file.txt	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/file.txt	Thu Jan 01 00:00:00 1970 +0000
+  @@ -6,6 +6,7 @@
+   5
+   6
+   7
+  -8
+  +y
+   9
+   10
+  +11
+Contrast with the `hg diff -c 7` version above: only the manual edit shows
+up, making it easy to identify changes someone is otherwise trying to sneak
+into a merge.
+  $ hg diff --merge -c 7
+  merging file.txt
+  diff -r 8ad85e839ba7 file.txt
+  --- a/file.txt	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/file.txt	Thu Jan 01 00:00:00 1970 +0000
+  @@ -9,3 +9,4 @@
+   y
+   9
+   10
+  +11
+
+Set up a conflict.
+  $ hg co ".^"
+  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ sed -e 's,^8$,z,' file.txt > file.txt.tmp
+  $ mv file.txt.tmp file.txt
+  $ hg ci -m 'conflicting edit: 8 to z'
+  created new head
+  $ echo "this file is new in p1 of the merge" > new-file-p1.txt
+  $ hg ci -Am 'new file' new-file-p1.txt
+  $ hg log -r . --template 'p1 will be rev {rev}\n'
+  p1 will be rev 9
+  $ hg co 5
+  1 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ echo "this file is new in p2 of the merge" > new-file-p2.txt
+  $ hg ci -Am 'new file' new-file-p2.txt
+  created new head
+  $ hg log -r . --template 'p2 will be rev {rev}\n'
+  p2 will be rev 10
+  $ hg co -- 9
+  2 files updated, 0 files merged, 1 files removed, 0 files unresolved
+  $ hg merge -r 10
+  merging file.txt
+  warning: conflicts while merging file.txt! (edit, then use 'hg resolve --mark')
+  1 files updated, 0 files merged, 0 files removed, 1 files unresolved
+  use 'hg resolve' to retry unresolved file merges or 'hg merge --abort' to abandon
+  [1]
+  $ hg revert file.txt -r .
+  $ hg resolve -ma
+  (no more unresolved files)
+  $ hg commit -m 'merge conflicted edit'
+Without --merge, it's a diff against p1
+  $ hg diff --no-merge -c 11
+  diff -r fd1f17c90d7c -r 5010caab09f6 new-file-p2.txt
+  --- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/new-file-p2.txt	Thu Jan 01 00:00:00 1970 +0000
+  @@ -0,0 +1,1 @@
+  +this file is new in p2 of the merge
+With --merge, it's a diff against the conflicted content.
+  $ hg diff --merge -c 11
+  merging file.txt
+  diff -r 5010caab09f6 file.txt
+  --- a/file.txt	Thu Jan 01 00:00:00 1970 +0000
+  +++ b/file.txt	Thu Jan 01 00:00:00 1970 +0000
+  @@ -6,12 +6,6 @@
+   5
+   6
+   7
+  -<<<<<<< local: fd1f17c90d7c - test: new file
+   z
+  -||||||| base
+  -8
+  -=======
+  -y
+  ->>>>>>> other: d9e7de69eac3 - test: new file
+   9
+   10
+
+There must _NOT_ be a .hg/merge directory leftover.
+  $ test ! -d .hg/merge
+(No output is expected)
   $ cd ..