Added failing test for relative remote URLs (#648)

[Yogu]

git.git supports relative URLs in remote configurations and resolves them relative to the working directory. libgit2 currently fails with "Unsupported URL protocol".

[Yogu]

I think the problem is transport_find_fn which normally chooses the file protocol if the file pointed to by the url exists, but it does not correctly handle relative paths. It passes the relative path directly to access, which will use the current working directory instead of the repository's working directory as a reference.

I'm not sure if this can be fixed without passing the repository's workdir to transport_find_fn.

[nulltoken]

/cc @carlosmn

[carlosmn]

It expects paths to be absolute, and I think we should keep it that way. It should be enough to detect a relative path in the remote and make give the absolute path to the transport finding function. Especially since the user can override it, so we should be making the input more predictable.