pack-objects: ignore ambiguous object warnings

A git push process runs several processes during its run, but one
includes git send-pack which calls git pack-objects and passes
the known have/wants into stdin using object ids. However, the
default setting for core.warnAmbiguousRefs requires git pack-objects
to check for ref names matching the ref_rev_parse_rules array in
refs.c. This means that every object is triggering at least six
"file exists?" queries.  When there are a lot of refs, this can
add up significantly! I observed a simple push spending three
seconds checking these paths.

The fix here is similar to 4c30d50 "rev-list: disable object/refname
ambiguity check with --stdin".  While the get_object_list() method
reads the objects from stdin, turn warn_on_object_refname_ambiguity
flag (which is usually true) to false.  Just for code hygiene, save
away the original at the beginning and restore it once we are done.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/pack-objects.c

@@ -2988,6 +2988,7 @@ static void get_object_list(int ac, const char **av)
 	struct rev_info revs;
 	char line[1000];
 	int flags = 0;
+	int save_warning;
 
 	init_revisions(&revs, NULL);
 	save_commit_buffer = 0;
@@ -2996,6 +2997,9 @@ static void get_object_list(int ac, const char **av)
 	/* make sure shallows are read */
 	is_repository_shallow(the_repository);
 
+	save_warning = warn_on_object_refname_ambiguity;
+	warn_on_object_refname_ambiguity = 0;
+
 	while (fgets(line, sizeof(line), stdin) != NULL) {
 		int len = strlen(line);
 		if (len && line[len - 1] == '\n')
@@ -3022,6 +3026,8 @@ static void get_object_list(int ac, const char **av)
 			die(_("bad revision '%s'"), line);
 	}
 
+	warn_on_object_refname_ambiguity = save_warning;
+
 	if (use_bitmap_index && !get_object_list_from_bitmap(&revs))
 		return;