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
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}