Use description for diff hash

Review requests with history uses full commit description. So
we need to hash it, too. Otherwise we do not update if
there is a commit-message-only change.

This fixes #7.

contrib/mercurial_git_push.py

@@ -573,6 +573,8 @@ def _get_hash(self, diffset_id):
             hasher = hashlib.sha256()
             for info in six.itervalues(self.diff_info_commits):
                 hasher.update(info.getHash(diffset_id).encode('utf-8'))
+            for changeset in self._changesets:
+                hasher.update(changeset.desc().encode('utf-8'))
             return hasher.hexdigest()
 
         return self.diff_info.getHash(diffset_id)