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/blob_gogit.go

@@ -14,7 +14,7 @@ import (
 
 // Blob represents a Git object.
 type Blob struct {
-	ID SHA1
+	ID ObjectID
 
 	gogitEncodedObj plumbing.EncodedObject
 	name            string