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_nogogit.go

@@ -5,7 +5,7 @@
 
 package git
 
-func (repo *Repository) getBlob(id SHA1) (*Blob, error) {
+func (repo *Repository) getBlob(id ObjectID) (*Blob, error) {
 	if id.IsZero() {
 		return nil, ErrNotExist{id.String(), ""}
 	}