Do not automatically load fugitive non has("win32unix")

For some reason :Git on a non cygwin initiated gvim was not working and
launching cmd.exe to do something.  This does not seem to be a problem if
manually sourcing fugitive.vim after gvim comes up.
Do not automatically load fugitive.

It seems somehow that com! in commands.vim and function! in function.vim do not
override the :Git command setup in fugitive.