tests/test-dirstate-backup.t

nodemap: double check the source docket when doing incremental update

In theory, the index will have the information we expect it to have. However by
security, it seems safer to double check that the incremental data are generated
from the data currently on disk.

Differential Revision: https://phab.mercurial-scm.org/D7890

Set up

  $ hg init repo
  $ cd repo

Try to import an empty patch

  $ hg import --no-commit - <<EOF
  > EOF
  applying patch from stdin
  abort: stdin: no diffs found
  [255]

No dirstate backups are left behind

  $ ls .hg/dirstate* | sort
  .hg/dirstate