commitablectx: fix the default phase

The code was ignoring the config option. Since the config option will control
the actual phase of the committed result, we better take it in account early.

Differential Revision: https://phab.mercurial-scm.org/D7726

mercurial/context.py

@@ -1439,7 +1439,7 b' class committablectx(basectx):'
         return b
 
     def phase(self):
-        phase = phases.draft  # default phase to draft
+        phase = phases.newcommitphase(self._repo.ui)
         for p in self.parents():
             phase = max(phase, p.phase())
         return phase