git: detect existing symlink hooks before overwriting - only update plain files If the existing hook is a symlink, the hook is not what we put in place, and we should stay away - no matter if it points at something that looks like a Kallithea hook.

# 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
.