New disk format pushed out to the git tree

Hello everyone,

Yan Zheng's latest commit redoes the extent back references to make them more efficient. This includes a new disk format, so any pulls from the git unstable trees will have a new disk format to them.

I've bumped the magic in both the kernel and progs repos, so you can't accidentally mount an FS with the wrong format.

I still hope to finish off compression this week, which will be yet another format bump.

-chris