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.

checkheads: use 'nodemap.get' to convert nodes to revs We are about to call 'torev' on node that might be locally missing. In this case, 'nodemap.revs' will return None (something valid in our usecase) while 'changelog.rev' would raise an exception. We make this change in a distinct changeset to show it does not impact the tests.

checkheads: pass "ispushed" function to the obsmarkers logic We are about to make "allfuturecommon" a set of revs instead of a set of nodes. The function updated in this patch do not needs to know about these details so we just pass it a 'ispushed(node)' function. This will simplify the next changeset.