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/repo_ref_nogogit.go

@@ -75,7 +75,7 @@ func (repo *Repository) GetRefsFiltered(pattern string) ([]*Reference, error) {
 		if pattern == "" || strings.HasPrefix(refName, pattern) {
 			r := &Reference{
 				Name:   refName,
-				Object: MustIDFromString(sha),
+				Object: repo.objectFormat.MustIDFromString(sha),
 				Type:   typ,
 				repo:   repo,
 			}