cygwin: define normcase spec and fallback

These will be used in upcoming patches to efficiently create a dirstate
foldmap.

The Cygwin normcase behavior is more complicated than just a simple lowercasing
or uppercasing. That's why we specify 'other'.

mercurial/posix.py

@@ -311,6 +311,9 b" if sys.platform == 'cygwin':"
 
         return encoding.upper(path)
 
+    normcasespec = encoding.normcasespecs.other
+    normcasefallback = normcase
+
     # Cygwin translates native ACLs to POSIX permissions,
     # but these translations are not supported by native
     # tools, so the exec bit tends to be set erroneously.