This halves the amount of code we have to take into account for dealing
with the config.
There is a slight change in the API. Config.get_multivar() returns an
iterator instead of a list, which lets us reuse code from the general
iterator and is closer to libgit2's API.
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.
