mergestate: don't pop stateextras when there are no conflicts on filemerge

Even if `filemerge.filemerge()` didn't result in conflicts, we should not remove
stateextras for a file since we now use that for more things than just merge
time information. We use stateextras to store information which is required to
be used by commit.

I tracked this down while finding why a patch of mine which adds more commit
related information does not work as expected and looses the extras in
mergestate.

Differential Revision: https://phab.mercurial-scm.org/D10148

mercurial/mergestate.py

@@ -382,7 +382,6 b' class _mergestate_base(object):'
         if merge_ret is None:
             # If return value of merge is None, then there are no real conflict
             del self._state[dfile]
-            self._stateextras.pop(dfile, None)
             self._dirty = True
         elif not merge_ret:
             self.mark(dfile, MERGE_RECORD_RESOLVED)