tests/test-dirstate-backup.t

grep: extract main search loop as searcher method

Still displayer part is in commands.grep(), the core grep logic is now
reusable. I'll revisit the displayer stuff later since it will be another
long series.

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