This updates to the latest clar which includes the helpers
`cl_assert_equal_s` and `cl_assert_equal_i`. Convert the code
over to use those and remove the old libgit2-only helpers.
This makes so much sense that I can't believe it hasn't been done
before. Kill the old `git_fbuffer` and read files straight into
`git_buf` objects.
Also: In order to fully support 4GB files in 32-bit systems, the
`git_buf` implementation has been changed from using `ssize_t` for
storage and storing negative values on allocation failure, to using
`size_t` and changing the buffer pointer to a magical pointer on
allocation failure.
Hopefully this won't break anything.