vcs: make hg get_changesets compatible with py3

If start was 0, we would end up with start_pos None and getting a compare with
None which would fail in py3.

Instead, make sure start_pos is None iff start is None.

Also, make the actual check more robust.

kallithea/lib/vcs/backends/hg/repository.py

@@ -509,11 +509,11 b' class MercurialRepository(BaseRepository'
         :param reversed: return changesets in reversed order
         """
         start_raw_id = self._get_revision(start)
-        start_pos = self.revisions.index(start_raw_id) if start else None
+        start_pos = None if start is None else self.revisions.index(start_raw_id)
         end_raw_id = self._get_revision(end)
-        end_pos = self.revisions.index(end_raw_id) if end else None
+        end_pos = None if end is None else self.revisions.index(end_raw_id)
 
-        if None not in [start, end] and start_pos > end_pos:
+        if start_pos is not None and end_pos is not None and start_pos > end_pos:
             raise RepositoryError("Start revision '%s' cannot be "
                                   "after end revision '%s'" % (start, end))