commit: clear mergestate also with --amend (issue6304)

The `hg commit --amend` uses the in-memory code, which naturally
doesn't touch the merge state (well, it shouldn't anyway; I think I've
fixed bugs in that area recently). We therefore need to clear the
mergestate after calling `repo.commitctx()` since we expect that from
`hg commit --amend`.

Differential Revision: https://phab.mercurial-scm.org/D8932

mercurial/cmdutil.py

@@ -3259,6 +3259,7 b' def amend(ui, repo, old, extra, pats, op'
         if opts.get(b'secret'):
             commitphase = phases.secret
         newid = repo.commitctx(new)
+        ms.reset()
 
         # Reroute the working copy parent to the new changeset
         repo.setparents(newid, nullid)

tests/test-amend.t

@@ -113,7 +113,6 b' Nothing changed'
   $ hg ci --amend --config experimental.evolution.allowunstable=True
   1 new orphan changesets
   $ hg resolve -l
-  R f
   $ cd ..
 #endif