dirstate.walk: move dirignore filter out of explicit walk code

This is a move towards factoring this code out into a separate function.

mercurial/dirstate.py

@@ -626,7 +626,6 b' class dirstate(object):'
                         results[nf] = None
                     if matchedir:
                         matchedir(nf)
-                    if not dirignore(nf):
                         wadd(nf)
                 elif kind == regkind or kind == lnkkind:
                     results[nf] = st
@@ -649,6 +648,9 b' class dirstate(object):'
                     else:
                         badfn(ff, inst.strerror)
 
+        work = [d for d in work if not dirignore(d)]
+        wadd = work.append
+
         # step 2: visit subdirectories
         while work:
             nd = work.pop()