completion: use __gitcomp_builtin in _git_merge

New completable options are:

--allow-unrelated-histories
--message=
--overwrite-ignore
--signoff
--strategy-option=
--summary
--verify

The variable $__git_merge_options remains because _git_pull() still
needs it. It will soon be gone after _git_pull() is updated.

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

contrib/completion/git-completion.bash

@@ -1768,8 +1768,13 @@ _git_merge ()
 
 	case "$cur" in
 	--*)
-		__gitcomp "$__git_merge_options
-			--rerere-autoupdate --no-rerere-autoupdate --abort --continue"
+		__gitcomp_builtin merge "--rerere-autoupdate
+				--no-rerere-autoupdate
+				--no-commit --no-edit --no-ff
+				--no-log --no-progress
+				--no-squash --no-stat
+				--no-verify-signatures
+				"
 		return
 	esac
 	__git_complete_refs