Abstract hash function usage (#28138)

Refactor Hash interfaces and centralize hash function. This will allow
easier introduction of different hash function later on.

This forms the "no-op" part of the SHA256 enablement patch.

modules/git/tree_blob_gogit.go

@@ -24,7 +24,7 @@ func (t *Tree) GetTreeEntryByPath(relpath string) (*TreeEntry, error) {
 			gogitTreeEntry: &object.TreeEntry{
 				Name: "",
 				Mode: filemode.Dir,
-				Hash: t.ID,
+				Hash: plumbing.Hash(t.ID.RawValue()),
 			},
 		}, nil
 	}