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>

packfile.h

@@ -154,10 +154,10 @@ int nth_packed_object_id(struct object_id *, struct packed_git *, uint32_t n);
 off_t nth_packed_object_offset(const struct packed_git *, uint32_t n);
 
 /*
- * If the object named sha1 is present in the specified packfile,
+ * If the object named by oid is present in the specified packfile,
  * return its offset within the packfile; otherwise, return 0.
  */
-off_t find_pack_entry_one(const unsigned char *sha1, struct packed_git *);
+off_t find_pack_entry_one(const struct object_id *oid, struct packed_git *);
 
 int is_pack_valid(struct packed_git *);
 void *unpack_entry(struct repository *r, struct packed_git *, off_t, enum object_type *, unsigned long *);