{fetch,receive}-pack: skip sha-1 integrity test on objects from new packnd/optim-connected

When we fetch or push, usually "git rev-list --verify-objects --not
--all --stdin" is used to make sure that all objects between existing
refs and new refs are good. This means no gaps in between, all objects
are well-formed, object content agrees with its sha-1 signature.

For the last one, --verify-objects calls check_sha1_signature() via
parse_object(). check_sha1_signature() is an expensive operation,
especially when new refs are far away from existing ones because all
objects in between are re-hashed. Because objects coming from the new
pack are already hashed by index-pack, we can trust their
integrity. The only objects left to check are existing ones in repo
but has no connection to any current refs.

Pass the new pack id down to--verify-objects and skip
check_sha1_signature() on objects from that pack.

As an (extreme) example, a repository is created with only one commit:
e83c516 (Initial revision of "git", the information manager from hell
- 2005-04-07). The rest of git.git is fetched on top. Without the
patch:

$ time git fetch file:///home/pclouds/w/git/.git
remote: Counting objects: 125656, done.
remote: Compressing objects: 100% (33280/33280), done.
remote: Total 125656 (delta 92585), reused 123464 (delta 90682)
Receiving objects: 100% (125656/125656), 34.60 MiB | 8.47 MiB/s, done.
Resolving deltas: 100% (92585/92585), done.
>From file:///home/pclouds/t/test/
 * branch            HEAD       -> FETCH_HEAD

real    1m30.437s
user    1m31.338s
sys     0m1.687s

With the patch:

$ time git fetch file:///home/pclouds/w/git/.git
remote: Counting objects: 125656, done.
remote: Compressing objects: 100% (33280/33280), done.
remote: Total 125656 (delta 92585), reused 123464 (delta 90682)
Receiving objects: 100% (125656/125656), 34.60 MiB | 7.86 MiB/s, done.
Resolving deltas: 100% (92585/92585), done.
>From file:///home/pclouds/t/test/
 * branch            HEAD       -> FETCH_HEAD

real    0m52.182s
user    0m53.151s
sys     0m1.465s

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

1 files changed, 5 insertions, 0 deletions

diff --git a/revision.c b/revision.c
index 18be62b316..b8aa4c7e15 100644
--- a/revision.c
+++ b/revision.c
@@ -1438,6 +1438,11 @@ static int handle_revision_opt(struct rev_info *revs, int argc, const char **arg
 		revs->tree_objects = 1;
 		revs->blob_objects = 1;
 		revs->verify_objects = 1;
+	} else if (!strcmp(arg, "--safe-pack")) {
+		if (strlen(argv[1]) != 40)
+			die("--safe-pack requires an SHA-1 as pack id, not %s", argv[1]);
+		strcpy(revs->safe_pack, argv[1]);
+		return 2;
 	} else if (!strcmp(arg, "--unpacked")) {
 		revs->unpacked = 1;
 	} else if (!prefixcmp(arg, "--unpacked=")) {