localrepo: refactor repo.branchheads() to use repo.branchmap().branchheads()

branchmap: introduce branchheads() method

localrepo: refactor repo.branchtip() to use repo.branchmap().branchtip()

branchmap: introduce branchtip() method

branchmap: cache open/closed branch head information This lets us determine the open/closed state of a branch without reading from the changelog (which can be costly over NFS and/or with many branches).

bookmarks: allow push -B to create a new remote head (issue2372) Push is currently allowed to create a new head if there is a remote bookmark that will be updated to point to the new head. If the bookmark is not known remotely then push aborts, even if a -B argument is about to push the bookmark. This change allows push to continue in this case. This does not require a wireproto force.