utils/fs: add test for create file in dir. (#95)
utils/fs: add test for create file in dir. (#95)

* Ensure that files can be created in a FS returned
  by Dir().
* Files created in a dir fs should have Filename()
  relative to dir's base.
mcuadros/go-git,alcortesm/go-git,src-d/go-git,fuchsia-mirror/third_party-github.com-src-d-go-git,alcortesm/go-git,go-git/go-git,go-git/go-git,fuchsia-mirror/third_party-github.com-src-d-go-git,mcuadros/go-git,src-d/go-git