xdiff: ignore empty lines before added functions with -W

xdiff: ignore empty lines before added functions with -W

If a new function and a preceding empty line is appended, diff -W shows
the previous function in full in order to provide context for that empty
line.  In most languages empty lines between sections are not
interesting in and off themselves and showing a whole extra function for
them is not what we want.

Skip empty lines when checking of the appended chunk starts with a
function line, thereby avoiding to extend the context just for them.

Helped-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>