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.go

@@ -10,7 +10,7 @@ import (
 )
 
 // NewTree create a new tree according the repository and tree id
-func NewTree(repo *Repository, id SHA1) *Tree {
+func NewTree(repo *Repository, id ObjectID) *Tree {
 	return &Tree{
 		ID:   id,
 		repo: repo,