rebase: avoid losing branch commits with --keepbranch (issue4835)

lock.release: do not unlink inherited locks This is part of a series that will allow locks to be inherited by subprocesses in limited circumstances. A subprocess unlinking a lock will lead to potential corruption from other concurrent processes.

lock: add a method to reacquire the lock after subprocesses exit This is part of a series that will allow locks to be inherited by subprocesses in limited circumstances.