pullrequests: refactor default title code This refactoring simply emphasizes that the origin reference used to generate the default title is not always the same as the one actually used to create the pull request; for PRs not from a branch head, the branch name is stored so it can be used for creating new iterations.

# 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
.