phases: explicitly evaluate list returned by map

On Python 3 map() returns a generator, which bool()s to true even if
it had an empty input set. Work around this by using list() on the
map() result.

mercurial/phases.py

@@ -213,7 +213,7 b' class phasecache(object):'
         self._phaserevs = revs
         self._populatephaseroots(repo)
         for phase in trackedphases:
-            roots = map(repo.changelog.rev, self.phaseroots[phase])
+            roots = list(map(repo.changelog.rev, self.phaseroots[phase]))
             if roots:
                 for rev in roots:
                     revs[rev] = phase