test: improve packfile.Encoder tests
test: improve packfile.Encoder tests

* Improve checks for encode/decode.

* Make it easier to extend this test with more
  storage backends.

mcuadros/go-git,fuchsia-mirror/third_party-github.com-src-d-go-git,src-d/go-git,go-git/go-git,src-d/go-git,go-git/go-git,fuchsia-mirror/third_party-github.com-src-d-go-git,mcuadros/go-git