Have RTD error if repo is unclean

.readthedocs.yml

@@ -9,7 +9,8 @@ build:
  # Avoid `git` treating the directory is dirty due to RTD changes.
  # ref: https://docs.readthedocs.io/en/stable/build-customization.html#avoid-having-a-dirty-git-index
  - git update-index --assume-unchanged continuous_integration/environment-doc.yml docs/conf.py
- - git status
+ # If we missed any, error and list the changed files.
+ - git diff --stat --exit-code
 
 sphinx:
  configuration: docs/conf.py