renderdiff: add golden repo support (#8689)

- Add GoldenManager to manage access to the repo containing the
  goldens
- Add tif comparison code
- Enable comparison by default for actual test

test/renderdiff/src/workflow_presubmit_msg.py

@@ -25,7 +25,8 @@ def get_last_commit():
   return (
     commit.split(' ')[1],
     title.strip(),
-    desc)
+    desc
+  )
 
 def sanitized_split(line, split_atom='\n'):
   return list(filter(lambda x: len(x) > 0, map(lambda x: x.strip(), line.split(split_atom))))