tests/test-dirstate-backup.t

patch: move yielding "\n" to the end of loop

The original logic makes it harder to reason about - it yields the "\n"
character belonging to the last line in the next loop iteration.

The new code is in theory a little bit slower. But is more readable. It
makes the following changes easier to read.

Differential Revision: https://phab.mercurial-scm.org/D3210

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