diff -r 4f0d4bc63b8a -r eb1b964b354b mercurial/phases.py
--- a/mercurial/phases.py	Wed Oct 18 12:36:23 2017 +0200
+++ b/mercurial/phases.py	Wed Oct 18 12:19:53 2017 -0500
@@ -660,7 +660,7 @@
     if old is None:
         old = ''
     else:
-        old = '%s' % old
+        old = phasenames[old]
     return {'node': node,
             'oldphase': old,
-            'phase': '%s' % new}
+            'phase': phasenames[new]}