manifest: add dirs() to manifestdict

manifest: add dirs() to manifestdict

Manifests should have a method of accessing its own dirs, not just the
context that references the manifest. This makes it easier for other
optimized versions of manifests to compute their own dirs in the most efficient
way.

--- a/mercurial/manifest.py	Wed Sep 17 22:34:34 2014 +0900
+++ b/mercurial/manifest.py	Fri Mar 13 15:19:54 2015 -0700
@@ -6,9 +6,10 @@
 # GNU General Public License version 2 or any later version.
 
 from i18n import _
-import mdiff, parsers, error, revlog, util
+import mdiff, parsers, error, revlog, util, scmutil
 import array, struct
 
+propertycache = util.propertycache
 
 class _lazymanifest(dict):
     """This is the pure implementation of lazymanifest.
@@ -147,6 +148,13 @@
         files.difference_update(m2)
         return files
 
+    @propertycache
+    def _dirs(self):
+        return scmutil.dirs(self)
+
+    def dirs(self):
+        return self._dirs
+
     def matches(self, match):
         '''generate a new manifest filtered by the match argument'''
         if match.always():