topic-revset: changectx creation in the revset
The current mutability filter is very expensive, we use a more direct and
efficent way to do so:
Before:
! wall 0.213261 comb 0.210000 user 0.210000 sys 0.000000 (best of 45)
After:
! wall 0.012195 comb 0.020000 user 0.020000 sys 0.000000 (best of 217)