clone: update to active bookmark, if set This cannot happen during a normal Mercurial clone, but is useful for extensions like hg-git where we know what to check out — the Git HEAD — but do not wish to track it later on using the `@` bookmark. Test Plan: I have not included an explicit test, as this is much more easily tested within the `hg-git` test suite. Differential Revision: https://phab.mercurial-scm.org/D9638