First up, KDevelop is currently a waste of time. KDevelop3 is a pain, and Kdevelop4 is still in beta and (I find) way too unstable for any serious work. If you need an c IDE Qt Creator (personal favourite, even for straight c) and NetBeans are the two best Linux ones (imo).I read the diff/sdiff/diff3 info page. Just to compare file differences... KDE has a nice file differences compare app as part of Kdevelop, which might be easyer for me (I think in visuals and not in a logical order, if you know what I mean ).
Secondly, you don't want to do your diff/patching with an IDE. The best visual thing by far is meld. It's much better than the KDE ones (and I say that as a Qt dev / KDE user). After making changes in your working directory, type "meld ." for an awesome view/editing/cleaning.
And thirdly, start small. You're wayyyyyyyyy jumping the gun. Start small. Make some X11 apps. Make some OpenGL ones. Learn C. And learn the public api's well first.
And lastly, (and i know this is going to start a flamewar) but I wouldn't start with git. I've been using it for four months (its our version control system at work, and I still get surprised issues with it. (Compared to svn which I learn in an hour ;D). If you need to version/branch your stuff just use copy and paste, or use something simple like svn for now. You should be focusing on the real stuff (code). Once you start doing more serious coding (with people) you'll see the real advantages of it.