commit: clear resolved mergestate even if working copy is clean

If the mergestate has resolved conflicts and a commit is successfully
created (either because there are changes in the working copy or
because ui.allowemptycommit=yes), we will also clear the merge
state. However, if the working copy is clean (and
ui.allowemptycommit=no), we leave the mergestate there. The user may
notice it in `hg resolve -l` output (but not in `hg status -v`
output). It's not clear how the user should clear it, but probably via
`hg co -C .`. It's also quite likely that they won't even notice it
and it will get cleared by a later `hg commit` (of unrelated
changes).

This patch makes it so that `hg commit` also clears resolved merge
conflicts even if the command doesn't end up writing a commit because
the working copy was empty. That's probably a little weird (commands
that abort should generally avoid changing the repo), but it still
seems mostly harmless, and it reduces the risk of more bugs like
https://bz.mercurial-scm.org/show_bug.cgi?id=5494. I just ran into a
version of that bug in the Evolve extension and that's what triggered
this series.

Differential Revision: https://phab.mercurial-scm.org/D8196

mercurial/localrepo.py

@@ -2956,6 +2956,7 b' class localrepository(object):'
                 or self.ui.configbool(b'ui', b'allowemptycommit')
             )
             if not allowemptycommit:
+                ms.reset()
                 return None
 
             if merge and cctx.deleted():

relnotes/next

@@ -61,6 +61,9 b''
  * `hg debugmergestate` output format changed. Let us know if that is
    causing you problems and we'll roll it back.
 
+ * Resolved merge conflicts are now cleared by `hg commit` even if the
+   working copy has no changes.
+
 
 == Internal API Changes ==
 

tests/test-update-branches.t

@@ -357,7 +357,6 b' Try to make empty commit while there are'
   nothing changed
   [1]
   $ hg resolve -l
-  R a
 
 Change/delete conflict is not allowed
   $ hg up -qC 3