tests: add coverage of Git pullrequests Git is so different than Mercurial that it is easier to use separate tests. Some of the tests that are relevant for Mercurial doesn't apply to the Git support. Also fix crash an odd corner case of creating PRs from a repo that has no branches at all.

# 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
.