checkheads: use a "lazyancestors" object for allfuturecommon Instead of walking all ancestors to compute the full set we now check membership lazily. This massively speed. On a million-ish revision repository, this remove 14 seconds from the push logic, making the checkheads function disappear from profile.

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.