cygwin: add cygwin specific normcase logic
in cygwin environment, mount point part of path is treated as case
sensitive, even though underlying NTFS is case insensitive.
this patch preserves mount point part of specified path, only if it is
absolute one.
there is no easy way to get list of current mount points from python
program, other than to execute "mount" external command, because
cygwin does not store current mount points into Unix/Linux like
/etc/XXXtab file.
so, this patch introduces cygwinmountpoints variable to list mount
points to be preserved case.
this allows some other extensions to customize mount point
configuration.
.. Common link and substitution definitions.
.. |hg(1)| replace:: **hg**\ (1)
.. _hg(1): hg.1.html
.. |hgrc(5)| replace:: **hgrc**\ (5)
.. _hgrc(5): hgrc.5.html
.. |hgignore(5)| replace:: **hgignore**\ (5)
.. _hgignore(5): hgignore.5.html