bash-completion: fix update) and ... in command list

bor@opensuse:~> tg
...       delete    files     log       patch     remote
base      depend    import    mail      prev      summary
create    export    info      next      push      update)

Fix pattern to filter out command names by making it less greedy.
Let it stop before first ")".

While on it, explicitly add "help" to command completion list.
It is listed now separately from other commands.

Signed-off-by: Andrey Borzenkov <arvidjaar@gmail.com>