Convert local `file:` URLs to paths before handing over to `git_clone`.

Fixes a bug where a locally-cloned remote would get a `file:` URL as it's origin URL, making the push machinery shrivel in fear.
Dot-syntaxify.
Use `-fileSystemRepresentation` here.
File reference URLs are file URLs; no need for a separate test.
Useless call; file reference URLs know how to path-ify themselves.
Move all GTRepositoryTests under GTRepositorySpec.
Test that the remote set by `+clone…` looks fine to libgit2.