Reduce the intf. of Reference and parameter [head_contents]/[contents] with an ['hash] type.
Reduce the intf. of Reference and parameter [head_contents]/[contents] with an ['hash] type.

We /de-functorize/ the Reference module and provide, as before, functions to
manipulate it. However, we consider a reference as a reference and we deleted
the bridge between a reference and a [Path.t]/[Fpath.t].

Indeed, such bridge is close to the underlying layout used to store Git
references.

mirage/ocaml-git,mirage/ocaml-git,mirage/ocaml-git