tests/test-dirstate-backup.t

scmutil: extract function that builds revset expr to select bookmark branch

This is needed to process "log -B" option properly. "log" options have to
be translated to a revset expression, not to an evaluated set.

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
  [10]

No dirstate backups are left behind

  $ ls .hg/dirstate* | sort
  .hg/dirstate