dirstate: clean up when restoring identical backups When a dirstate backup is restored, it is possible that no actual changes to the dirstate have been made. In this case, the backup is still a hardlink to the original dirstate. Unfortunately, `os.rename` silently fails (nothing happens, and no error occurs) when `src` and `dst` are hardlinks to the same file. As a result, the backup is left lying around. Over time, these files accumulate. When restoring dirstate backups, check if the backup and the dirstate are the same file, and if so, just delete the backup. Differential Revision: https://phab.mercurial-scm.org/D1201