This change removes the last bit of deprecated API for 5.0: the old get_orig_commit_id method. This was more complex than the rest because the mercurial backend still used it in its diff parsers (both mercurial and git modes). I've updated the hg parsers to instead set the parent commit ID on the parsed diff change, as well as set the ``uses_commit_ids_as_revisions`` flag. This also required updating a unit test which wasn't set up quite right. Now that commits and parent commits are done per file instead of per diff (as per all the other machinery in `create_filediffs`), we can't have a parent diff that specifies different files than the diff and expect the parent diff commit ID to get set correctly. Testing Done: Ran unit tests.