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>