It's Magit! A Git porcelain inside Emacs.
Magit is an interface to the version control system Git, implemented as an Emacs package. Magit aspires to be a complete Git porcelain. While we cannot (yet) claim that Magit wraps and improves upon each and every Git command, it is complete enough to allow even experienced Git users to perform almost all of their daily version control tasks directly from within Emacs. While many fine Git clients exist, only Magit and Git itself deserve to be called porcelains.
If you are new to Magit, then either one of the following two articles should help understanding how it differs from other Git clients.
If you are completely new to Magit, then this article is a good visual introduction.
Almost everything that you see in Magit can be acted on by pressing some key, but that's not obvious from just seeing how Magit looks. The screenshots and accompanying text of this article explain how to perform a variety of actions on Magit's output.
Magit differs significantly from other Git interfaces, and its advantages are not immediately obvious simply from looking at a few screenshots as presented in the preceding article.
This article discusses Magit's properties in somewhat more abstract terms.
When something doesn't work as expected then please first see the FAQ. Then also try the list of open issues and use the search box at the top of that page to find older related issues. You should also consult the manual and ask a general-purpose search engine.
If that doesn't answer your question, then ask for help on the Emacs Stackexchange site or the mailing list. We only use the GitHub issue tracker for feature requests and bug reports, so please don't ask for help there.
Magit was started by Marius Vollmer, and is now maintained by Jonas Bernoulli, Kyle Meyer, and Noam Postavsky. Other former maintainers are Nicolas Dudebout, Peter J. Weisberg, Phil Jackson, Rémi Vanicat, and Yann Hodique. Many more people have contributed code and suggested features.
Over the years a lot of people supported development financially, including the 1987 backers of the 2017 crowdfunding campaign.
Thanks to all of you, may (the history of) the source be with you!