sha1_file: do not access pack if unneeded

Currently, regardless of the contents of the "struct object_info" passed
to sha1_object_info_extended(), that function always accesses the
packfile whenever it returns information about a packed object, since it
needs to populate "u.packed".

Add the ability to pass NULL, and use NULL-ness of the argument to
activate an optimization in which sha1_object_info_extended() does not
needlessly access the packfile. A subsequent patch will make use of this
optimization.

A similar optimization is not made for the cached and loose cases as it
would not cause a significant performance improvement.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1_file.c

@@ -2977,12 +2977,16 @@ static int sha1_loose_object_info(const unsigned char *sha1,
 
 int sha1_object_info_extended(const unsigned char *sha1, struct object_info *oi, unsigned flags)
 {
+	static struct object_info blank_oi = OBJECT_INFO_INIT;
 	struct pack_entry e;
 	int rtype;
 	const unsigned char *real = (flags & OBJECT_INFO_LOOKUP_REPLACE) ?
 				    lookup_replace_object(sha1) :
 				    sha1;
 
+	if (!oi)
+		oi = &blank_oi;
+
 	if (!(flags & OBJECT_INFO_SKIP_CACHED)) {
 		struct cached_object *co = find_cached_object(real);
 		if (co) {
@@ -3020,6 +3024,13 @@ int sha1_object_info_extended(const unsigned char *sha1, struct object_info *oi,
 		}
 	}
 
+	if (oi == &blank_oi)
+		/*
+		 * We know that the caller doesn't actually need the
+		 * information below, so return early.
+		 */
+		return 0;
+
 	rtype = packed_object_info(e.p, e.offset, oi);
 	if (rtype < 0) {
 		mark_bad_packed_object(e.p, real);