5.2 is in Alpha.
Package developers working in 5-1 should continue to work in 5-1 until after 5.2.0 is released. At that point, they can merge their work from 5-1 to 5-2 and start testing against 5-2 core.
Package developers starting new packages should use 5-2.
The branch does include the whole tree.
For core, you should commit bug-fixes to 5.2. You should only commit 5.1 core fixes for critical core bugs.
For packages, commit bug fixes according to the same rules as development.
