git: make sure _check_url only accept the protocols accepted by is_valid_repo_uri Avoid unnecessary flexibility, ambiguity, and complexity. The file protocol was never used. But when cloning existing managed repos, is_valid_repo_url would be skipped and _check_url would be called with absolute paths.

# requirements.txt file for use as "pip install -r requirements.txt" as a
# readthedocs compatible alternative to "pip install -e ." which is a working
# alternative to "setup.py develop" which doesn't work with Mercurial 3.7
.