Pass the contents of the buffer containing the git_oid to bytes() so
build a raw representation of its contents. Using bytes(ffi.buffer(...))
returns the representation of the buffer.
This moves enough code into python with CFFI to pass the test_remotes
unit tests. There is no credentials support yet.
There is a small change in the return value of Remote.fetch() in that we
now return a TransferProgress object instead of extracting a few values
into a dictionary.
