vfs: add listdir for os.listdir in vfs

vfs: add listdir for os.listdir in vfs

This patch adds listdir() in vfs to use os.listdir().

--- a/mercurial/scmutil.py	Mon Jun 23 09:24:56 2014 -0400
+++ b/mercurial/scmutil.py	Fri Jun 20 21:18:14 2014 +0530
@@ -184,6 +184,9 @@
     def lstat(self, path=None):
         return os.lstat(self.join(path))
 
+    def listdir(self, path=None):
+        return os.listdir(self.join(path))
+
     def makedir(self, path=None, notindexed=True):
         return util.makedir(self.join(path), notindexed)