bundle2: add a "check:heads" handler This part is intended to hold the same role as the `heads` argument of the unbundle function. The client fill it with the known heads at bundle time and the server will abort if its heads changed.

repoview: make the conversion from node to rev explicit while computing hidden You cannot use `repo[...]` lookup here. 1. It is slow 2. It is very likely to trigger a hidden computation itself, entering an infinite loop.