dirstate._walkexplicit: don't bother normalizing '.'

The overwhelmingly common case is running commands like 'hg diff' with no
arguments. Therefore the only file that'll be listed is the root directory.
Normalizing that's just a waste of time.

This means that for a plain 'hg diff' we'll never need to construct the
foldmap, saving us a significant chunk of time.

On case-insensitive HFS+ on OS X, for a large repository with over 200,000
files, this brings down 'hg diff' from 2.97 seconds to 2.36.

mercurial/dirstate.py

@@ -634,7 +634,9 b' class dirstate(object):'
 
         alldirs = None
         for ff in files:
-            if normalize:
+            # constructing the foldmap is expensive, so don't do it for the
+            # common case where files is ['.']
+            if normalize and ff != '.':
                 nf = normalize(ff, False, True)
             else:
                 nf = ff