tests: only diff the `Cargo.lock` in `test-check-cargo-lock.t` While not perfect (you could be running tests while developing with a justified modification to `Cargo.lock`), that use-case is small enough that I don't think it's really going to be an issue. This stops the test from failing for unrelated changes when running it locally. Differential Revision: https://phab.mercurial-scm.org/D9170

#require cargo test-repo
  $ . "$TESTDIR/helpers-testrepo.sh"
  $ cd "$TESTDIR"/../rust

Check if Cargo.lock is up-to-date. Will fail with a 101 error code if not.

  $ cargo check --locked --all --quiet

However most CIs will run `cargo build` or similar before running the tests, so we need to check if it was modified

  $ testrepohg diff Cargo.lock