dirstate.walk: use the file foldmap to normalize

Computing the set of directories in the dirstate is expensive. It turns out
that it isn't necessary for operations like 'hg status' at all.

Why? Consider the file 'foo/bar' on disk, which is represented in the dirstate
as 'FOO/BAR'.

On 'hg status', we'd walk down the directory tree, coming across 'foo' first.

Before: we'd normalize 'foo' to 'FOO', then add 'FOO' to our visited stack.
We'd then visit 'FOO', finding the file 'bar'. We'd normalize 'FOO/bar' to
'FOO/BAR', then add it to the results dict.

After: we wouldn't normalize 'foo' at all. We'd add it to our visited stack,
then visit 'foo', finding the file 'bar'. We'd normalize 'foo/bar' to
'FOO/BAR', then add it to the results dict.

So whether we normalize intermediate directories or not actually makes no
difference in most cases.

The only case where normalization matters at all is if a file is replaced with
a directory with the same case-folded name. In that case we can do a relatively
cheap file normalization instead and still get away with not computing the set
of directories.

This is a nice boost in status performance. On OS X with case-insensitive HFS+,
for a large repo with over 200,000 files, this brings down 'hg status' from
4.00 seconds to 3.62.

mercurial/dirstate.py

@@ -744,10 +744,10 b' class dirstate(object):'
             skipstep3 = True
 
         if not exact and self._checkcase:
-            normalize = self._normalize
+            normalizefile = self._normalizefile
             skipstep3 = False
         else:
-            normalize = None
+            normalizefile = None
 
         # step 1: find all explicit files
         results, work, dirsnotfound = self._walkexplicit(match, subrepos)
@@ -772,8 +772,11 b' class dirstate(object):'
                     continue
                 raise
             for f, kind, st in entries:
-                if normalize:
-                    nf = normalize(nd and (nd + "/" + f) or f, True, True)
+                if normalizefile:
+                    # even though f might be a directory, we're only interested
+                    # in comparing it to files currently in the dmap --
+                    # therefore normalizefile is enough
+                    nf = normalizefile(nd and (nd + "/" + f) or f, True, True)
                 else:
                     nf = nd and (nd + "/" + f) or f
                 if nf not in results: