completion: treat "branch -D" the same way as "branch -d"

The former offers not just branches but tags as completion
candidates.

Mimic how "branch -d" limits its suggestion to branch names.

Reported-by: Paul Jolly <paul@myitcv.io>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1438,7 +1438,7 @@ _git_branch ()
 	while [ $c -lt $cword ]; do
 		i="${words[c]}"
 		case "$i" in
-		-d|--delete|-m|--move)	only_local_ref="y" ;;
+		-d|--delete|-D|-m|--move)	only_local_ref="y" ;;
 		-r|--remotes)		has_r="y" ;;
 		esac
 		((c++))