phases: move root phase assignment to it's own function

This moves the initial root phase assignment to it's own function. Future
patches which make phase calculations lazy will use this function to pre-fill
certain phases which can be deduced from the roots.

mercurial/phases.py

@@ -167,6 +167,8 b' class phasecache(object):'
         if self._phaserevs is None:
             repo = repo.unfiltered()
             revs = [public] * len(repo.changelog)
+            self._phaserevs = revs
+            self._populatephaseroots(repo)
             for phase in trackedphases:
                 roots = map(repo.changelog.rev, self.phaseroots[phase])
                 if roots:
@@ -174,11 +176,21 b' class phasecache(object):'
                         revs[rev] = phase
                     for rev in repo.changelog.descendants(roots):
                         revs[rev] = phase
-            self._phaserevs = revs
         return self._phaserevs
+
     def invalidate(self):
         self._phaserevs = None
 
+    def _populatephaseroots(self, repo):
+        """Fills the _phaserevs cache with phases for the roots.
+        """
+        cl = repo.changelog
+        phaserevs = self._phaserevs
+        for phase in trackedphases:
+            roots = map(cl.rev, self.phaseroots[phase])
+            for root in roots:
+                phaserevs[root] = phase
+
     def phase(self, repo, rev):
         # We need a repo argument here to be able to build _phaserevs
         # if necessary. The repository instance is not stored in