zsh_completion: add -b/--branch and -B/--bookmark(s) flags properly
_hg_branch_bmark_opts used to add these two flags, but had the same
descriptions for the flags regardless of what command took them and didn't
allow specifying flags more than once (no '*' at the start). Even more
importantly, it assumed that -B was always expecting an argument (i.e.
--bookmark=foo), but in case of incoming and outgoing it's not so (--bookmarks
Differential Revision: https://phab.mercurial-scm.org/D4612