A remote can have a multitude of refspecs. Up to now our git_remote's
have supported a single one for each fetch and push out of simplicity
to get something working.
Let the remotes and internal code know about multiple remotes and get
the tests passing with them.
Instead of setting a refspec, the external users can clear all and add
refspecs. This should be enough for most uses, though we're still
missing a querying function.
Passing NULL is non-sensical. The error message leaves to be desired,
though, as it leaks internal implementation details. Catch it at the
`git_config_set_string` level and set an appropriate error message.
