This crowd-funded campaign has been trying to raise $2,500 USD over the course of one month so the developer is able to work full-time on diving into Mesa for two weeks. There's sixteen days left in the campaign and he's up to $1,727. For more details, see his IndieGoGo campaign.
Anyhow, on Saturday morning the developer, Timothy Arceri, published his experimental "mesa-debug" branch for implementing the KHR_debug extension. The code is still not yet fully working (with the campaign not even being over yet) and there also aren't yet any Piglit regression tests published. The GitHub project page is here and the current changes are queued up in this branch.
The GL_KHR_debug extension allows OpenGL to notify applications about different events, in order to assist application/game developers in debugging and performance profiling/optimizing of their applications. The GL_KHR_debug extension is required as part of core OpenGL 4.2.