checkheads: use "revnum" in the "allfuturecommon" set
The obsolete post-processing needs to know the extend of the pushed set. The
way it is implemented is... suboptimal. It build a full set of all nodes in the
pushset and it does so using changectx. We have much better API for this now.
The simplest is to use the existing lazy ancestors computation. That logic uses
revnum and not node (for good reason) so we start with updating the
post-processing code to handle a "allfuturecommon" set containing revision
numbers.
local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed