add cache list command to pub

utils/pub/git_source.dart

Index: utils/pub/git_source.dart
===================================================================
--- utils/pub/git_source.dart	(revision 19938)
+++ utils/pub/git_source.dart	(working copy)
@@ -116,6 +116,10 @@
     });
   }
 
+  Future<List<Package>> getCachedPackages() {
+    //TODO(keertip): to be impelemented.

[Bob Nystrom, 2013/03/13 17:44:07]

Remove the entire implementation (so that the throw from the base class will
still happen) and just add a TODO in the class body:

// TODO(keertip): Implement getCachedPackages().

[keertip, 2013/03/13 18:09:48]

Done.

+  }
+  
   /// Ensure that the canonical clone of the repository referred to by [id] (the
   /// one in `<system cache>/git/cache`) exists and is up-to-date. Returns a
   /// future that completes once this is finished and throws an exception if it