git: implement stub prefetch_parents dirstate method A recent change (35b255e474d9) introduced this new required dirstate method but didn't update the git extension.

rust: leverage .expect() in place of .unwrap() + inline comment For a better error indication in case we made a mistake.

rust: fix false comment about mpsc::Sender We need Sync to share the Sender reference across threads.

merge with stable

relnotes: advertize the possibility to use rust I think the rust work may have been mentioned in the release notes, but if so only in passing, and not as an invitation to try it out. I think the next version is a decent time to do this, because the rust doesn't come with performance regressions AFAIK, speeds up status noticeably when it applies, which is the case for most invocations of status, and doesn't have the undesirable restriction of regex around empty patterns anymore. I am cheating a bit, because I'm giving numbers for `hg status` in mozilla-central, but they have one hgignore pattern that uses lookaround, ".vscode/(?!extensions\.json|tasks\.json", which I took out as it would cause a fallback to python when unknown files are requested. But it seems that they could express their hgignore differently if they were so inclined. Not sure if there are limitation other than linux-only that I am not thinking of but would be worth mentioning upfront, to avoid disappointing users? Differential Revision: https://phab.mercurial-scm.org/D8604

rust: add a pointer for profiling to the README As figuring out how to get useful profiles is not obvious. Differential Revision: https://phab.mercurial-scm.org/D8603

rust: update the mention of hgcli in rust/README.rst This may not be exactly right, but it's better than before. Differential Revision: https://phab.mercurial-scm.org/D8602