bash completion: complete refs for git-grep

bash completion: complete refs for git-grep

Before the --, always attempt ref completion.  This helps with
entering the <treeish> arguments to git-grep.  As a bonus, you can
work around git-grep's current lack of --all by hitting M-*, ugly as
the resulting command line may be.

Strictly speaking, completing the regular expression argument (or
option argument) makes no sense.  However, we cannot prevent _all_
completion (it will fall back to filenames), so we dispense with any
additional complication to detect whether the user still has to enter
a regular expression.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>