store: document the decoding discrepancy in store.py

This will help future people that might be looking into this.

Differential Revision: https://phab.mercurial-scm.org/D11220

mercurial/store.py

@@ -569,6 +569,11 b' class encodedstore(basicstore):'
         self.vfs = vfsmod.filtervfs(vfs, encodefilename)
         self.opener = self.vfs
 
+    # note: topfiles would also need a decode phase. It is just that in
+    # practice we do not have any file outside of `data/` that needs encoding.
+    # However that might change so we should probably add a test and encoding
+    # decoding for it too. see issue6548
+
     def datafiles(self, matcher=None):
         for t, a, b, size in super(encodedstore, self).datafiles():
             try: