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.

mercurial/discovery.py

@@ -248,11 +248,10 b' def _headssummary(pushop):'
 
     # If there are no obsstore, no post processing are needed.
     if repo.obsstore:
-        allmissing = set(outgoing.missing)
-        cctx = repo.set('%ld', outgoing.common)
-        allfuturecommon = set(c.rev() for c in cctx)
         torev = repo.changelog.rev
-        allfuturecommon.update(torev(m) for m in allmissing)
+        futureheads = set(torev(h) for h in outgoing.missingheads)
+        futureheads |= set(torev(h) for h in outgoing.commonheads)
+        allfuturecommon = repo.changelog.ancestors(futureheads, inclusive=True)
         for branch, heads in sorted(headssum.iteritems()):
             remoteheads, newheads, unsyncedheads, placeholder = heads
             result = _postprocessobsolete(pushop, allfuturecommon, newheads)