tests/test-dirstate-backup.t

rust-matchers: remove default implementations for `Matcher` trait

We don't expect a whole lot of matchers to be defined, and this makes it more
obvious what a matcher does by reading its `impl Matcher for FooMatcher`.

This patch has the added benefit of fixing the `AlwaysMatcher`, its `matches`
function differs from the former default.

Differential Revision: https://phab.mercurial-scm.org/D7255

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