Sorry if this functionality already exists, or has been discussed already, but I couldn't find anything...
IMHO, Diffusion should allow to edit code directly in the browser, for example with an editor like [[ http://ace.c9.io/ | Ace ]]. I know I can link an external editor (which is a nice feat btw), but there are cases where you can't or don't want to clone a repo just to make a few tweaks.
In a first version, this could be as simple as an "edit this file" button, which starts the editor and asks for a commit message when saving, simply committing to whatever branch one is currently browsing. As an extension, there could be an option to create a new branch, to support PR-style workflows like on Github.