config: allow setting multivars when none exist yet

Adding a multivar when there are no variables with that name set
should set the variable instead of failing.
remote: handle multiple refspecs

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.
remote: allow querying for refspecs

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.
refspec: unify the string and parsed data

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.
Merge pull request #1493 from carlosmn/remotes

Revamp the refspec handling