The Gallium3D mega drivers work is just like the other Mesa mega drivers model. This refactoring of the Mesa code is to basically shove multiple drivers/components into a single .so library file. By doing this, there's potential minor performance improvements through allowing the compiler to apply link-time optimizations more broadly, and it can also reduce disk space.
With Emil's work committed hours ago, the final DRI target in the Gallium3D code-base has been converted into a single DRI mega-driver library. Prior to this was the merging of some DRI targets and a number of other changes by Emil.
These changes will be found in the Mesa 10.3 release due out later this summer.