Gitless manual install documentation for MacOS 10.11 (El Capitan)

Install and link Python2 (optional, but recommended).

brew install python
brew link python

Install and link Python3.

brew install python3
brew link python3

Install and link Git.

brew install git
brew link git

Prepend PATH in ~/.bash_profile with /usr/local.

vim ~/.bash_profile
export PATH=/usr/local/bin:$PATH
source ~/.bash_profile

Install libgit2, a dependency of PyGit2 that pip doesn't install.

brew install libgit2

Install PyGit2.

pip3.5 install pygit2

Download gitless tarball and manually install (via Python easy_install).

wget -c https://github.com/sdg-mit/gitless/archive/v0.8.4.tar.gz 
tar xzvf v0.8.4.tar.gz
python3.5 setup.py install