submodule: suppress checking for file name and ref ambiguity for object ids

The argv argument of collect_changed_submodules() contains only object ids
(the objects references of all the refs).

Notify setup_revisions() that the input is not filenames by passing
assume_dashdash, so it can avoid redundant stat for each ref.

Also suppress refname_ambiguity flag to avoid filesystem lookups for
each object. Similar logic can be found in cat-file, pack-objects and more.

This change reduces the time for git fetch in my repo from 25s to 6s.

Signed-off-by: Orgad Shaneh <orgads@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

submodule.c

@@ -840,9 +840,16 @@ static void collect_changed_submodules(struct repository *r,
 {
 	struct rev_info rev;
 	const struct commit *commit;
+	int save_warning;
+	struct setup_revision_opt s_r_opt = {
+		.assume_dashdash = 1,
+	};
 
+	save_warning = warn_on_object_refname_ambiguity;
+	warn_on_object_refname_ambiguity = 0;
 	repo_init_revisions(r, &rev, NULL);
-	setup_revisions(argv->nr, argv->v, &rev, NULL);
+	setup_revisions(argv->nr, argv->v, &rev, &s_r_opt);
+	warn_on_object_refname_ambiguity = save_warning;
 	if (prepare_revision_walk(&rev))
 		die(_("revision walk setup failed"));