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
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).