This document is a stub and should be improved.
Greasemonkey source code is managed via Git. Git is a very different version control system, and you should learn how to use it.
Links:
Most importantly, all independent changes should be isolated into separate branches. Any given branch should be clearly explained and proposed for inclusion in Greasemonkey via the greasemonkey-dev mailing list, or the issue ticket if there is an appropriate one.
When proposing a change to the source code, change exactly one thing. Do not refactor some existing code, and add new code for a new feature at the same time.
Please include the entire development community when hacking on Greasemonkey. Discussion on the mailing list is highly encouraged. Discussion on the appropriate issue is often equally appropriate.