packfile: use object_id in find_pack_entry_one()

The main function we use to search a pack index for an object is
find_pack_entry_one(). That function still takes a bare pointer to the
hash, despite the fact that its underlying bsearch_pack() function needs
an object_id struct. And so we end up making an extra copy of the hash
into the struct just to do a lookup.

As it turns out, all callers but one already have such an object_id. So
we can just take a pointer to that struct and use it directly. This
avoids the extra copy and provides a more type-safe interface.

The one exception is get_delta_base() in packfile.c, when we are chasing
a REF_DELTA from inside the pack (and thus we have a pointer directly to
the mmap'd pack memory, not a struct). We can just bump the hashcpy()
from inside find_pack_entry_one() to this one caller that needs it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

t/helper/test-find-pack.c

@@ -40,7 +40,7 @@ int cmd__find_pack(int argc, const char **argv)
 		die("cannot parse %s as an object name", argv[0]);
 
 	for (p = get_all_packs(the_repository); p; p = p->next)
-		if (find_pack_entry_one(oid.hash, p)) {
+		if (find_pack_entry_one(&oid, p)) {
 			printf("%s\n", p->pack_name);
 			actual_count++;
 		}