tests/test-dirstate-backup.t

help: assigning categories to existing commands

I'm separating this into its own commit so people can bikeshed over the actual
categorization (vs the support for categories). These categories are based on
the help implementation we've been using internally at Google, and have had
zero complaints.

Differential Revision: https://phab.mercurial-scm.org/D5067

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