dirstate.walk: maintain a list of dirs not found

Upcoming patches will factor out the walk over explicit files done in step 1.
This helps us get there.

mercurial/dirstate.py

@@ -607,6 +607,7 b' class dirstate(object):'
         results['.hg'] = None
 
         # step 1: find all explicit files
+        dirsnotfound = []
         for ff in files:
             if normalize:
                 nf = normalize(normpath(ff), False, True)
@@ -642,6 +643,7 b' class dirstate(object):'
                         if fn.startswith(prefix):
                             if matchedir:
                                 matchedir(nf)
+                            dirsnotfound.append(nf)
                             skipstep3 = False
                             break
                     else: