OS X: try cheap ascii .lower() in normcase before making full unicode dance

This is similar to what is done in encoding.lower, introduced in e7a5733d533f.

This has been seen making 'hg up' and 'hg st' in a 50000+ files repo 13%
faster.

This might make Mercurial slightly slower for users who mainly use non-ASCII
filenames. That is a reasonable trade-off.

mercurial/posix.py

@@ -194,6 +194,11 @@ if sys.platform == 'darwin':
     import fcntl # only needed on darwin, missing on jython
 
     def normcase(path):
+        try:
+            path.decode('ascii') # throw exception for non-ASCII character
+            return path.lower()
+        except UnicodeDecodeError:
+            pass
         try:
             u = path.decode('utf-8')
         except UnicodeDecodeError: