completion: use __gitcomp_builtin in _git_commit

The new comletable options are:

--branch
--gpg-sign
--long
--no-post-rewrite
--null
--porcelain
--status

--allow-empty is no longer completable because it's a hidden option
since 4741edd549 (Remove deprecated OPTION_BOOLEAN for parsing arguments
- 2013-08-03)

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

contrib/completion/git-completion.bash

@@ -1353,16 +1353,7 @@ _git_commit ()
 		return
 		;;
 	--*)
-		__gitcomp "
+		__gitcomp_builtin commit "--no-edit --verify"
-			--all --author= --signoff --verify --no-verify
-			--edit --no-edit
-			--amend --include --only --interactive
-			--dry-run --reuse-message= --reedit-message=
-			--reset-author --file= --message= --template=
-			--cleanup= --untracked-files --untracked-files=
-			--verbose --quiet --fixup= --squash=
-			--patch --short --date --allow-empty
-			"
 		return
 	esac