completion: use __gitcomp_builtin in _git_merge_base

The new completion option is --all.

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

contrib/completion/git-completion.bash

@@ -1798,7 +1798,7 @@ _git_merge_base ()
 {
 	case "$cur" in
 	--*)
-		__gitcomp "--octopus --independent --is-ancestor --fork-point"
+		__gitcomp_builtin merge-base
 		return
 		;;
 	esac