dirstate._walkexplicit: indicate root as '.', not ''

'.' is the canonical way to represent the root, and it's apparently the only
transformation that normpath makes.

mercurial/dirstate.py

@@ -629,7 +629,7 b' class dirstate(object):'
             j += 1
 
         if not files or '.' in files:
-            files = ['']
+            files = ['.']
         results = dict.fromkeys(subrepos)
         results['.hg'] = None