scmutil: document that bookmarkrevs() ignores non-head bookmark branch
"- ancestors(head() and not bookmark(%s))" excludes the bookmarked branch
itself if bookmark(%s) is not a head. I'm a bit surprised by this behavior
while writing "log -B" tests, so let's document it.
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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
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commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).