git/odb/pack: implement v2 object searching

[ttaylorr]

This pull request implements object searching for v1 index files, and is the third in the series started by #2420.

It implements the v2 index lookup, similar to that implemented in #2421.

The v2 index format varies slightly from v1. Instead of placing the object offset and object name next to each other, they are split into separate "tables". v2 also knows how to store a cyclic redundancy check for each object, as well how to encode 8-byte unsigned offsets.

More specifically, the structure is as follows:

1. A magic 4-byte header
2. A 4-byte version tag (network order)
3. 256 4-byte network order unsigned integers comprising the L1 fanout table.
4. The 20-byte object names, in order.
5. 4-byte entries for each object giving the CRC in network byte order.
6. 4-byte "small" offsets. If the most significant bit of this is set (i.e., x&0x8000000 > 1), the rest of the offset encodes the location of an 8-byte offset in the same file.

The v2Search function introduced in this pull request understands the above format and can search through it.

---

/cc @git-lfs/core @peff
/cc #2415