ancestor: use heapreplace() in place of heappop/heappush()

This should be slightly faster.

Overall perfancestors result::

                 cpython           nginx             mercurial
  -------------  ----------------  ----------------  ----------------
  b6db2e80a9ce^  0.103461          0.006303          0.035716
  8eb2145ff0fb   0.192307 (x1.86)  0.012115 (x1.92)  0.052135 (x1.46)
  this patch     0.139986 (x1.35)  0.006389 (x1.01)  0.037176 (x1.04)

mercurial/ancestor.py

@@ -264,6 +264,7 b' def _lazyancestorsiter(parentrevs, initr'
     seen = {nullrev}
     heappush = heapq.heappush
     heappop = heapq.heappop
+    heapreplace = heapq.heapreplace
     see = seen.add
 
     if inclusive:
@@ -294,8 +295,7 b' def _lazyancestorsiter(parentrevs, initr'
             if current - p1 == 1:
                 visit[0] = -p1
             else:
-                heappop(visit)
-                heappush(visit, -p1)
+                heapreplace(visit, -p1)
             see(p1)
         else:
             heappop(visit)