grep: -W: skip trailing empty lines at EOF, too

grep: -W: skip trailing empty lines at EOF, too

4aa2c4753d (grep: -W: don't extend context to trailing empty lines,
2016-05-28) stopped showing empty lines at the end of function context
when using -W.  Do the same for trailing empty lines at the end of
files, for consistency -- it doesn't matter whether a function section
is ended by the next function or the end of the file.

Test it by adding a trailing empty line to the file used by the test
"grep -W" and leave its expected output the same.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>