tests/test-dirstate-backup.t

formatter: populate ctx from repo and node value

This will basically replace the fm.contexthint() API. I originally thought
this would be too complicated, and I wrote 8399438bc7ef "formatter: provide
hint of context keys required by template" because of that. However, I had
to add a similar mechanism for fctx templates, and the overall machinery
became way simpler than my original patch.

The test output slightly changed as {author} is no longer available in
the {manifest} context, which isn't the point this test targeted on.

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