Revert "Add a Git supplemental configuration for Cygwin"

This reverts commit c893a7f65433d6cdbb1d5727488451bad39538cf.

Because this little hack won't work because the included Git is so old it
doesn't include support for the include.path option. Cygwin installs are
uncommon enough that just upgrading Git is probably easiest. Cygwin Ports
makes this somewhat easier, at least, compiling from source isn't needed.