normcase: for darwin, use fast ASCII lower

Constructing the foldmap is much faster on OS X now. For a large real-world
repo, hg perfdirstatefoldmap:

before: wall 0.805278 comb 0.800000 user 0.790000 sys 0.010000 (best of 13)
after:  wall 0.399708 comb 0.410000 user 0.390000 sys 0.020000 (best of 25)

This is a nice boost to 'hg status', especially with the third-party hgwatchman
extension enabled (which eliminates stat calls). For the above repo, 'hg
status' goes from 1.17 seconds to 0.74.

mercurial/posix.py

@@ -217,8 +217,7 b" if sys.platform == 'darwin':"
         '''
 
         try:
-            path.decode('ascii') # throw exception for non-ASCII character
-            return path.lower()
+            return encoding.asciilower(path)  # exception for non-ASCII
         except UnicodeDecodeError:
             pass
         try: