These are fairly basic as the decoder is also exercised by the fragment
parsing tests, but they cover some errror cases that may not be covered
otherwise.
Git uses a unique Base85 encoding with different characters than the
ascii85 encoding implemented by Go, so add a custom decoding function.
Once decoded, use zlib instead of the raw DEFLATE algorithm to
decompress the data.
These issues were caught by some basic parsing tests which are added
here as well.