...Solved (but please update Docs - this took me quite some time!)
The Wiki Page reads:

git clone git://github.com/tbranyen/nodegit.git
$ cd nodegit
$ npm run-script gen && npm install

this fails, complaining about missing ejs (on all machines/distris)

so I run:

npm i ejs &&  npm run-script gen && npm install

Compile succeeds!!!, BUT:

npm test

always gave me:
Cannot find module './build/Debug/nodegit'

I had cmake 2.8 installed (centos here)
but cmake --version told me 2.6 was in use.

Solved by:

ln -s /usr/bin/cmake28 /usr/bin/cmake