Emacs. Git. [https://lists.gnu.org/archive/html/emacs-devel/2014-11/msg00681.html Emacs and Git].

{{{
From: Eric S. Raymond
Subject: New Git repository is up.
Date: Wed, 12 Nov 2014 22:12:55 -0500

I repacked, push-mirrored it, then recloned.  It looks correct as far as
I can tell in the limited time I've had to inspect it.

If we later find conversion flaws, they should be fixable with much less
effort than the initial conversion.

Commits are open. Have at it.
}}}

[http://lars.ingebrigtsen.no/2014/11/13/welcome-new-emacs-developers/ Welcome, New Emacs Developers] -- a blog post by LarsMagneIngebrigtsen explaining how to check it out, how to take a look at the wishlist, how to contribute patches.

The wiki also hosts a very readable introduction of the workflow: GitForEmacsDevs.

If you're building on a Mac, remember to use ##./configure --with-ns## or you'll get an executable requiring X11 or working on the terminal, only.
You must run "make install" in order to test the built application.
The installed application will go to ##nextstep/Emacs.app## and can be
run or moved from there.
The application will be fully self-contained.