diff: suppress `merging foo` output lines when performing a merge diff

Differential Revision: https://phab.mercurial-scm.org/D9939

mercurial/commands.py

@@ -2576,7 +2576,9 b' def diff(ui, repo, *pats, **opts):'
                 },
                 b'diff --merge',
             ):
+                repo.ui.pushbuffer()
                 mergemod.merge(pctx2, wc=wctx)
+                repo.ui.popbuffer()
             ctx1 = wctx
         else:
             ctx1 = ctx2.p1()

tests/test-diff-change.t

@@ -197,7 +197,6 b" must be similar to 'hg diff --change 5':"
 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.
@@ -228,7 +227,6 b' Contrast with the `hg diff -c 7` version'
 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
@@ -277,7 +275,6 b" Without --merge, it's a diff against p1"
   +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