show-branch: handle [] globs as well.

Earlier only '?' and '*' signalled the command that what the
user has given is a glob pattern.  This prevented us to say:

	$ git show-branch 'v0.99.[0-3]'

Now we notice '[' as well, so the above would work.

Signed-off-by: Junio C Hamano <junkio@cox.net>