discovery: abort also when pushing multiple headed new branch Before this patch, pushing with --new-branch permits to create multiple headed branch on the destination repository. But permitting to create new branch should be different from permitting to create multiple heads on branch. This patch prevents from careless pushing multiple headed new branch, and requires --force to push such branch forcibly.

branchmap: stop looking for stripped branch Since repoview in 2.5 we do not make special call to `branchmap` when stripping. We just recompute the branchmap from a lower subset that still has valid branchmap. So I'm dropping this dead code.