revset: fix ancestors(null) to include null revision (issue4512)

Since 13c0327eeb6f, null parent is explicitly excluded. So, there is no reason
to have nullrev in the initial seen set.

mercurial/revset.py

@@ -31,7 +31,7 b' def _revancestors(repo, revs, followfirs'
             revsnode = revqueue.popleft()
             heapq.heappush(h, -revsnode)
 
-        seen = set([node.nullrev])
+        seen = set()
         while h:
             current = -heapq.heappop(h)
             if current not in seen:

tests/test-log.t

@@ -635,6 +635,20 b' log -f -r 1:tip'
   
 
 
+log -f -r null
+
+  $ hg log -f -r null
+  changeset:   -1:000000000000
+  user:        
+  date:        Thu Jan 01 00:00:00 1970 +0000
+  
+  $ hg log -f -r null -G
+  o  changeset:   -1:000000000000
+     user:
+     date:        Thu Jan 01 00:00:00 1970 +0000
+  
+
+
 log -r .  with two parents
 
   $ hg up -C 3

tests/test-revset.t

@@ -477,6 +477,10 b' Test explicit numeric revision'
   hg: parse error: rev expects a number
   [255]
 
+Test null revision
+  $ log 'ancestors(null)'
+  -1
+
   $ log 'outgoing()'
   8
   9