nix-prefetch-git: Convert relative submodule URLS to absolute URLS

nix-prefetch-git does not convert relative submodule urls into absolute
urls based on the parent's origin. This patch adds support for
repositories which are using the relative url syntax.
Merge pull request #1482 from wkennington/master.git

nix-prefetch-git: Convert relative submodule URLS to absolute URLS