At its core, the config is a key-value storage with peculiar key
equality rules. Make it behave more like a python dictionary and return
the key on iteration.
Reconcile the changes between the ffi changes upstream with the changes
to libgit2 in the dev branch.
Conflicts:
src/config.c
src/options.c
src/refspec.c
src/remote.c
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.
