shallow.c: remove useless code

Some context before we talk about the removed code.

This paint_down() is part of step 6 of 58babfff (shallow.c: the 8 steps
to select new commits for .git/shallow - 2013-12-05). When we fetch from
a shallow repository, we need to know if one of the new/updated refs
needs new "shallow commits" in .git/shallow (because we don't have
enough history of those refs) and which one.

The question at step 6 is, what (new) shallow commits are required in
other to maintain reachability throughout the repository _without_
cutting our history short? To answer, we mark all commits reachable from
existing refs with UNINTERESTING ("rev-list --not --all"), mark shallow
commits with BOTTOM, then for each new/updated refs, walk through the
commit graph until we either hit UNINTERESTING or BOTTOM, marking the
ref on the commit as we walk.

After all the walking is done, we check the new shallow commits. If we
have not seen any new ref marked on a new shallow commit, we know all
new/updated refs are reachable using just our history and .git/shallow.
The shallow commit in question is not needed and can be thrown away.

So, the code.

The loop here (to walk through commits) is basically

1.  get one commit from the queue
2.  ignore if it's SEEN or UNINTERESTING
3.  mark it
4.  go through all the parents and..
5a. mark it if it's never marked before
5b. put it back in the queue

What we do in this patch is drop step 5a because it is not
necessary. The commit being marked at 5a is put back on the queue, and
will be marked at step 3 at the next iteration. The only case it will
not be marked is when the commit is already marked UNINTERESTING (5a
does not check this), which will be ignored at step 2.

But we don't care about refs marking on UNINTERESTING. We care about the
marking on _shallow commits_ that are not reachable from our current
history (and having UNINTERESTING on it means it's reachable). So it's
ok for an UNINTERESTING not to be ref-marked.
Reported-by: NRasmus Villemoes <rv@rasmusvillemoes.dk>
Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

shallow.c

@@ -434,12 +434,8 @@ static void paint_down(struct paint_info *info, const unsigned char *sha1,
 			    oid_to_hex(&c->object.oid));
 
 		for (p = c->parents; p; p = p->next) {
-			uint32_t **p_refs = ref_bitmap_at(&info->ref_bitmap,
-							  p->item);
 			if (p->item->object.flags & SEEN)
 				continue;
-			if (*p_refs == NULL || *p_refs == *refs)
-				*p_refs = *refs;
 			commit_list_insert(p->item, &head);
 		}
 	}