Play with go-vcs vfs rooted paths.

Related to https://github.com/sourcegraph/go-vcs/issues/23.
Compare UnsafeReflectValue code paths.

Related to
https://github.com/davecgh/go-spew/commit/128854244a4ac24b718da3d5f79cf5
541c81ee56#commitcomment-8495157.