git/odb/pack: implement v1 object searching

[ttaylorr]

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

The main goal of this pull request is to implement a function that the IndexVersion.Search() function can call into, as:

func (i *IndexVersion) Search(idx *Index, name []byte, at uint64) (*IndexEntry, int, error) {
        switch i {
        case V1:
                return v1IndexSearch(idx, name, at)
        }

        panic(...)
}

This implements the search functionality (i.e., find an object, or return a comparison value of where to search next) for V1 pack files.

I estimate that it's unlikely LFS will parse many v1 packfiles during regular usage. Git stopped writing v1 packfiles near git/git@c553ca2, which was first released in the v1.5.2 series, which was released earlier than our minimum supported version of Git.

That being said, I think it's important for LFS to be able to parse all packfiles, since we won't have git-cat-file(1) to fallback on if we can't parse an earlier packfile version.

V1 packfiles have the following structure:

1. 256 4-byte network order unsigned integers, comprising the L1 fanout table.
2. One 24 byte entry per object, composed of a 4-byte network byte ordered unsigned integer (the object offset) and 20 bytes for the object name.

The v1Search function understands the above structure and does the appropriate search/comparison operation.

---

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