localrepo: extract bookmarkheads method to bookmarks.py...
localrepo: extract bookmarkheads method to bookmarks.py
This method is only used internally by destutil, and it's obscure
enough I'm willing to just move it without a deprecation warning,
especially since the new method has more constrained functionality.
Design-wise I'd also like to get active bookmark handling folded into
the bookmark store, so that we don't squirrel away an extra attribute
for the active bookmark on the repository object.