phases: replace magic number by constant

Differential Revision: https://phab.mercurial-scm.org/D8695

mercurial/phases.py

@@ -886,7 +886,7 b' def newcommitphase(ui):'
 
 def hassecret(repo):
     """utility function that check if a repo have any secret changeset."""
-    return bool(repo._phasecache.phaseroots[2])
+    return bool(repo._phasecache.phaseroots[secret])
 
 
 def preparehookargs(node, old, new):