phases: write phaseroots deterministically

self.phaseroots is a list of sets of binary nodes.

Let's sort the nodes before writing so the phaseroots file is
written out deterministically.

Differential Revision: https://phab.mercurial-scm.org/D2468

mercurial/phases.py

@@ -326,7 +326,7 b' class phasecache(object):'
 
     def _write(self, fp):
         for phase, roots in enumerate(self.phaseroots):
-            for h in roots:
+            for h in sorted(roots):
                 fp.write('%i %s\n' % (phase, hex(h)))
         self.dirty = False