pack-revindex: read multi-pack reverse indexes

Implement reading for multi-pack reverse indexes, as described in the
previous patch.

Note that these functions don't yet have any callers, and won't until
multi-pack reachability bitmaps are introduced in a later patch series.
In the meantime, this patch implements some of the infrastructure
necessary to support multi-pack bitmaps.

There are three new functions exposed by the revindex API:

  - load_midx_revindex(): loads the reverse index corresponding to the
    given multi-pack index.

  - midx_to_pack_pos() and pack_pos_to_midx(): these convert between the
    multi-pack index and pseudo-pack order.

load_midx_revindex() and pack_pos_to_midx() are both relatively
straightforward.

load_midx_revindex() needs a few functions to be exposed from the midx
API. One to get the checksum of a midx, and another to get the .rev's
filename. Similar to recent changes in the packed_git struct, three new
fields are added to the multi_pack_index struct: one to keep track of
the size, one to keep track of the mmap'd pointer, and another to point
past the header and at the reverse index's data.

pack_pos_to_midx() simply reads the corresponding entry out of the
table.

midx_to_pack_pos() is the trickiest, since it needs to find an object's
position in the psuedo-pack order, but that order can only be recovered
in the .rev file itself. This mapping can be implemented with a binary
search, but note that the thing we're binary searching over isn't an
array of values, but rather a permuted order of those values.

So, when comparing two items, it's helpful to keep in mind the
difference. Instead of a traditional binary search, where you are
comparing two things directly, here we're comparing a (pack, offset)
tuple with an index into the multi-pack index. That index describes
another (pack, offset) tuple, and it is _those_ two tuples that are
compared.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

midx.c

@@ -47,11 +47,22 @@ static uint8_t oid_version(void)
 	}
 }
 
+static const unsigned char *get_midx_checksum(struct multi_pack_index *m)
+{
+	return m->data + m->data_len - the_hash_algo->rawsz;
+}
+
 static char *get_midx_filename(const char *object_dir)
 {
 	return xstrfmt("%s/pack/multi-pack-index", object_dir);
 }
 
+char *get_midx_rev_filename(struct multi_pack_index *m)
+{
+	return xstrfmt("%s/pack/multi-pack-index-%s.rev",
+		       m->object_dir, hash_to_hex(get_midx_checksum(m)));
+}
+
 static int midx_read_oid_fanout(const unsigned char *chunk_start,
 				size_t chunk_size, void *data)
 {