Accept any value for the remote's url, including an empty string which
we used to reject as invalid configuration.
This is not quite what git does (although it has its own problems with
such configurations) and it makes it harder to fix the issue, by not
letting the user modify it.
As we already need to check for a valid URL when we try to connect to
the network, let that perform the check, as we don't need to do it
anywhere else.
It used to be separate as an attempt to make the querying easier, but
it didn't work out that way, so put all the data together.
Add git_refspec_string() as well to get the original string, which is
now stored alongside the independent parts.
Introduce git_remote_{fetch,push}_refspecs() to get a list of refspecs
from the remote and rename the refspec-adding functions to a less
silly name.
Use this instead of the vector index hacks in the tests.
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.
