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.