store: move top file walk to a separate function

Some extensions find it useful to be able to walk the non-data files in the
repo, so I'm moving that part of the walk to a separate function.

In particular, this allows an extension to interact with only the non-filelog
store data (for instance, when cloning everything but filelogs).

mercurial/store.py

@@ -322,13 +322,16 b' class basicstore(object):'
     def datafiles(self):
         return self._walk('data', True)
 
+    def topfiles(self):
+        # yield manifest before changelog
+        return reversed(self._walk('', False))
+
     def walk(self):
         '''yields (unencoded, encoded, size)'''
         # yield data files first
         for x in self.datafiles():
             yield x
-        # yield manifest before changelog
-        for x in reversed(self._walk('', False)):
+        for x in self.topfiles():
             yield x
 
     def copylist(self):