t7810: add test for grep -W and trailing empty context lines

Add a test demonstrating that git grep -W prints empty lines following
the function context we're actually interested in.  The modified test
file makes it necessary to adjust three unrelated test cases.

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