tests/test-check-cargo-lock.t

merge: move the filtering of ambiguous files to a dedicated function

I have multiple reasons:
	- The body of `_update` is way too long
	- This adds typing which will help our tooling and brains understand this
	  code more easily
	- This function will get more nested and complex in the next patch

I've taken the liberty of rewrapping and typo-passing the docstring.

#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