Change revlog.heads to walk the revision graph using revision numbers

On the kernel repo:
$ hg heads -q
           before    after
RevlogNG    1.11     0.52
Revlogv0    0.80     0.69

Since the current code for tags has to find all the heads of the repo,
this also helps there:
$ hg tags
           before    after
RevlogNG    2.35     1.76
Revlogv0    2.04     1.90

mercurial/revlog.py

@@ -713,19 +713,19 b' class revlog(object):'
         """
         if start is None:
             start = nullid
-        reachable = {start: 1}
-        heads = {start: 1}
         startrev = self.rev(start)
+        reachable = {startrev: 1}
+        heads = {startrev: 1}
 
+        parentrevs = self.parentrevs
         for r in xrange(startrev + 1, self.count()):
-            n = self.node(r)
-            for pn in self.parents(n):
-                if pn in reachable:
-                    reachable[n] = 1
-                    heads[n] = 1
-                if pn in heads:
-                    del heads[pn]
-        return heads.keys()
+            for p in parentrevs(r):
+                if p in reachable:
+                    reachable[r] = 1
+                    heads[r] = 1
+                if p in heads:
+                    del heads[p]
+        return [self.node(r) for r in heads]
 
     def children(self, node):
         """find the children of a given node"""