xdiff -W: relax end-of-file function detection

xdiff -W: relax end-of-file function detection

When adding a new function to the end of a file, it's enough to know
that 1) the addition is at the end of the file; and 2) there is a
function _somewhere_ in there.

If we had simply been changing the end of an existing function, then we
would also be deleting something from the old version.

This fixes the case where we add e.g.

	// Begin of dummy
	static int dummy(void)
	{
	}

to the end of the file.

Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Acked-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>