Use vfs.FileSystem interface instead of vcs.FileSystem.

That should be all the references as far as I can find.

However, I see this repo vendors its dependencies. I've never used the `--copy=true` mode of `godep` before, and I tried to get it to work now, but I wasn't sure how to get it to pick up dependencies from all subpackages instead of just the Go package in the root folder... Any ideas?
What commit was this? It doesn't seem to exist on GitHub.

https://github.com/sourcegraph/go-vcs/commit/537a57f9fab2b54695c9ac835a8fd12c64bd5e5e