Re: [PATCH 7/9] 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, but rather a _permutation_.

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 <redacted>
---
 midx.c          |  11 +++++
 midx.h          |   6 +++
 pack-revindex.c | 112 ++++++++++++++++++++++++++++++++++++++++++++++++
 pack-revindex.h |  46 ++++++++++++++++++++
 packfile.c      |   3 ++
 5 files changed, 178 insertions(+)

diff --git a/midx.c b/midx.c
index bf258c4fde..12bfce8bb1 100644
--- a/midx.c
+++ b/midx.c

@@ -48,11 +48,22 @@ 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;