tests/test-dirstate-backup.t

tags: behave better if a tags cache entry is partially written

This is done by discarding any partial cache entry, instead of
filling the partial cache entry with 0xff before.

Differential Revision: https://phab.mercurial-scm.org/D8095

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