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_test.go

@@ -61,7 +61,7 @@ func TestRepository_GetBlob_NoId(t *testing.T) {
 	defer r.Close()
 
 	testCase := ""
-	testError := fmt.Errorf("Length must be 40: %s", testCase)
+	testError := fmt.Errorf("length must be 40: %s", testCase)
 
 	blob, err := r.GetBlob(testCase)
 	assert.Nil(t, blob)