merge: add a files method to the mergestate class

merge: add a files method to the mergestate class

This will be used in the upcoming shelve extension.

--- a/mercurial/merge.py	Sun Jun 02 18:09:06 2013 -0500
+++ b/mercurial/merge.py	Mon Jun 03 17:20:37 2013 -0700
@@ -61,6 +61,8 @@
         l.sort()
         for f in l:
             yield f
+    def files(self):
+        return self._state.keys()
     def mark(self, dfile, state):
         self._state[dfile][0] = state
         self._dirty = True