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_blob.go

@@ -5,7 +5,7 @@ package git
 
 // GetBlob finds the blob object in the repository.
 func (repo *Repository) GetBlob(idStr string) (*Blob, error) {
-	id, err := NewIDFromString(idStr)
+	id, err := repo.objectFormat.NewIDFromString(idStr)
 	if err != nil {
 		return nil, err
 	}