merge: infer the "other" changeset when falling back to v1 format

When we have to fallback to the old version of the file, we infer the
"other" from current working directory parent. The same way it is currently done
in the resolve command. This is know to have shortcoming… but we cannot do
better from the data contained in the old file format. This is actually the
motivation to add this new file format.

mercurial/merge.py

@@ -74,6 +74,10 b' class mergestate(object):'
         for rev in v1records:
             if rev not in allv2:
                 # v1 file is newer than v2 file, use it
+                # we have to infer the "other" changeset of the merge
+                # we cannot do better than that with v1 of the format
+                mctx = self._repo[None].parents()[-1]
+                v1records.append(('O', mctx.hex()))
                 return v1records
         else:
             return v2records