topics: add t0 and b0 to the stack t0 or b0 will be the base of the stack and it's the parent of t1 or b1. The cool thing about this is that if you update to t0 using `hg up t0` or do `hg prev` on t1, you will be updated to t0 with the current topic preserved. This patch adds t0 to stack and implement the preserving topic case for t0 while using `hg update`.