match: strengthen visit_children_set invariant, Recursive means "all files"
My previous interpretation of "Recursive" was too relaxed: I thought it
instructed the caller to do something like this:
> you can stop calling `visit_children_set` because you'll need to descend into
> every directory recursively, but you should still check every file if it
> matches or not
Whereas the real instruction seems to be:
> I guarantee that everything in this subtree matches, you can stop
> querying the matcher for all files and dirs altogether.
The evidence to support this:
- the test actually passes with the stronger invariant, revealing no
exceptions from this rule
- the implementation of `visit_children_set` for `DifferenceMatcher`
clearly relies on this requirement, so it must hold for that not to
lead to bugs.
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).