phase: use `trackedphases` in `_getphaserevsnative`

phase: use `trackedphases` in `_getphaserevsnative`

Instead of manually listing secret and draft, simply use the `trackedphases`
constant. The constant is already used by `_getphaserevsnative`.

--- a/mercurial/phases.py	Fri Aug 24 17:45:46 2018 -0400
+++ b/mercurial/phases.py	Tue Jun 05 18:16:07 2018 +0200
@@ -272,19 +272,16 @@
         repo = repo.unfiltered()
         cl = repo.changelog
         self._phasesets = [set() for phase in allphases]
-        roots = pycompat.maplist(cl.rev, self.phaseroots[secret])
-        if roots:
-            ps = set(cl.descendants(roots))
-            for root in roots:
-                ps.add(root)
-            self._phasesets[secret] = ps
-        roots = pycompat.maplist(cl.rev, self.phaseroots[draft])
-        if roots:
-            ps = set(cl.descendants(roots))
-            for root in roots:
-                ps.add(root)
-            ps.difference_update(self._phasesets[secret])
-            self._phasesets[draft] = ps
+        lowerroots = set()
+        for phase in reversed(trackedphases):
+            roots = pycompat.maplist(cl.rev, self.phaseroots[phase])
+            if roots:
+                ps = set(cl.descendants(roots))
+                for root in roots:
+                    ps.add(root)
+                ps.difference_update(lowerroots)
+                lowerroots.update(ps)
+                self._phasesets[phase] = ps
         self._loadedrevslen = len(cl)
 
     def loadphaserevs(self, repo):