completion: use __gitcomp_builtin in _git_fetch

New completable options:

--deepen=
--ipv4
--ipv6
--jobs=
--multiple
--progress
--refmap=
--shallow-exclude=
--shallow-since=
--update-head-ok

Since _git_pull() needs fetch options too, $__git_fetch_options
remains. This variable 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

@@ -1462,7 +1462,7 @@ _git_fetch ()
 		return
 		;;
 	--*)
-		__gitcomp "$__git_fetch_options"
+		__gitcomp_builtin fetch "--no-tags"
 		return
 		;;
 	esac