rhg: add sparse support

rhg: add debugrhgsparse command to help figure out bugs in rhg

rhg-status: extract a function for printing pattern file warnings This will be reused for the warnings produced by the sparse file parsing functions.

rust-filepatterns: allow overriding default syntax This will be used when parsing pattern files other than .hgignore like the sparse spec.

rhg: don't fallback if `strip` or `rebase` are activated Neither of these extensions do anything other than add commands, so ignoring them opens up more of the test suite to rhg.

rhg: fallback in `debugdata` if repo has `narrow` Narrow uses ellipsis nodes and debugdata does not understand them yet.

rust-status: expose DifferenceMatcher from Rust to Python

rust-matchers: implement DifferenceMatcher This can be generated by the sparse matcher.

rhg: support "!" syntax for disabling extensions This makes it so that calls in test-log.t do not fall back immediately because of the disabled extension, instead going through the CLI parsing code, which breaks because of invalid UTF-8 in a flag. I *think* clap 3.x+ supports this? I'm not sure, and we have to upgrade the minimum Rust version to use clap 3.x anyway which is out of scope for this series, so let's just kick that can down the road a little bit.

revlog: finer computation of "issnapshot" If the parent had an empty diff, we skip of it to compute a diff against the parent base. This create shorter and simpler chain. However these case could be wrongly detected as snapshot. So we improve the code doing this detection. In practice nobody care as when tried on a copy of mozilla-try and we got the same number of snapshot (1315) in both case. Performance where equivalent.