log: add bookmark option to "hg log"
Before pushing a bookmark with "hg push origin -B 'my-topic'", it is useful to inspect
the list of commits that are ancestors of the bookmark.
By relying on scmutil.bookmarkrevs(), "hg log -B topic" has the same bookmark semantics
found in other commands like hg export, hg email, hg strip.
Differential Revision: https://phab.mercurial-scm.org/D9341
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