completion: complete merge-base options

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1499,6 +1499,12 @@ _git_mergetool ()
 
 _git_merge_base ()
 {
+	case "$cur" in
+	--*)
+		__gitcomp "--octopus --independent --is-ancestor --fork-point"
+		return
+		;;
+	esac
 	__gitcomp_nl "$(__git_refs)"
 }