git-completion.bash: stash-show: complete $__git_diff_common_options

Signed-off-by: Robert Karszniewicz <avoidr@posteo.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
index f363554..83bc646 100644
--- a/contrib/completion/git-completion.bash
+++ b/contrib/completion/git-completion.bash
@@ -3020,7 +3020,10 @@
 		list,--*)
 			__gitcomp "--name-status --oneline --patch-with-stat"
 			;;
-		show,--*|branch,--*)
+		show,--*)
+			__gitcomp "$__git_diff_common_options"
+			;;
+		branch,--*)
 			;;
 		branch,*)
 			if [ $cword -eq 3 ]; then