completion: use __gitcomp_builtin in _git_tag

The new completable options are:

--color
--format=
--ignore-case

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -3002,11 +3002,7 @@ _git_tag ()
 
 	case "$cur" in
 	--*)
-		__gitcomp "
+		__gitcomp_builtin tag
-			--list --delete --verify --annotate --message --file
-			--sign --cleanup --local-user --force --column --sort=
-			--contains --no-contains --points-at --merged --no-merged --create-reflog
-			"
 		;;
 	esac
 }