phases: fix `nullid` reference in newheads stable

phases: fix `nullid` reference in newheads

Spotted by Yuya Nishihara.

--- a/mercurial/phases.py	Mon Aug 20 16:33:48 2018 -0400
+++ b/mercurial/phases.py	Sat Aug 18 10:24:16 2018 +0200
@@ -673,7 +673,7 @@
     rev = cl.nodemap.get
     if not roots:
         return heads
-    if not heads or heads == [nullrev]:
+    if not heads or heads == [nullid]:
         return []
     # The logic operated on revisions, convert arguments early for convenience
     new_heads = set(rev(n) for n in heads if n != nullid)