Add --staged to bash completion for git diff

The --staged option (synonym for --cached) isn't listed in the
completion choices for git diff.  This tiny patch adds it.

Trivially-Acked-by: Shawn O. Pearce <spearce@spearce.org>

Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -899,7 +899,7 @@ _git_diff ()
 	local cur="${COMP_WORDS[COMP_CWORD]}"
 	case "$cur" in
 	--*)
-		__gitcomp "--cached --pickaxe-all --pickaxe-regex
+		__gitcomp "--cached --staged --pickaxe-all --pickaxe-regex
 			--base --ours --theirs
 			$__git_diff_common_options
 			"