Cygwin port added a "not quite correct but a lot faster and good
enough for many lstat() calls that are only used to see if the
working tree entity matches the index entry" lstat() emulation some
time ago, and it started biting us in places.  This removes it and
uses the standard lstat() that comes with Cygwin.

Recent topic that uses lstat on packed-refs file is broken when
this cheating lstat is used, and this is a simplest fix that is
also the cleanest direction to go in the long run.

* rj/cygwin-clarify-use-of-cheating-lstat:
  cygwin: Remove the Win32 l/stat() implementation