git/odb/pack: implement *ChainBase for unpacking base components

[ttaylorr]

This pull request provides the first of two implementations of the Chain interface:

In the following two pull requests in this series, two implementations of Chain will be provided: *ChainBase and *ChainDelta, which will decompress and apply patch deltas respectively.

*ChainBase handles the first kind of component of the delta-base chains found in Git packfiles: the base. A base encodes some base object's data in a zlib-compressed format, to which delta operations can be applied (patched).

Since the *ChainBase must handle zlib-compressed data, we run into the challenge of converting the mmap(2)'d *os.File (given as an io.ReaderAt) into an io.Reader that package compress/zlib can read from. This is done using the *OffsetReaderAt type, which implements the io.Reader interface by encoding an offset into an io.ReaderAt, and updating the offset each time a read is performed.

Because we cannot readily determine the length of the compressed object data in a packfile, we must take into account the greedy behavior from package compress/zlib. One approach to dealing with this is to find the next object, and limit reads from going beyond that. However, since we are not sharing *OffsetReaderAt instances with more than one Chain implementation, we can read past the end of the compressed data, and compress/zlib will simply ignore the extraneous bytes. Here is an example.

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