Merge branch 'fc/remote-testgit-feature-done'

In the longer term, tightening rules is a good thing to do, and
because nobody who has worked in the remote helper area seems to be
interested in reviewing this, I would assume they do not think
such a retroactive tightening will affect their remote helpers.  So
let's advance this topic to see what happens.

* fc/remote-testgit-feature-done:
  remote-testgit: properly check for errors

diff --cc git-remote-testpy.py
Simple merge

diff --cc t/t5800-remote-testpy.sh
Simple merge