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
is self-sufficient).
Differential Revision: https://phab.mercurial-scm.org/D4612
$ rm -rf a
$ hg init a
$ cd a
$ echo a > a
$ hg ci -Am0
adding a
$ hg tag t1 # 1
$ hg tag --remove t1 # 2
$ hg co 1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg tag -f -r0 t1
$ hg tags
tip 3:a49829c4fc11
t1 0:f7b1eb17ad24
$ cd ..