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

  $ hg init repo
  $ cd repo

  $ touch foo
  $ hg ci -Am 'add foo'
  adding foo

  $ hg up -C null
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved

this should be stored as a delta against rev 0

  $ echo foo bar baz > foo
  $ hg ci -Am 'add foo again'
  adding foo
  created new head

  $ hg debugindex foo
     rev    offset  length  ..... linkrev nodeid       p1           p2 (re)
       0         0       0  .....       0 b80de5d13875 000000000000 000000000000 (re)
       1         0      13  .....       1 0376abec49b8 000000000000 000000000000 (re)

  $ cd ..