grep: -W: don't extend context to trailing empty lines

Empty lines between functions are shown by grep -W, as it considers them
to be part of the function preceding them.  They are not interesting in
most languages.  The previous patches stopped showing them for diff -W.

Stop showing empty lines trailing a function with grep -W.  Grep scans
the lines of a buffer from top to bottom and prints matching lines
immediately.  Thus we need to peek ahead in order to determine if an
empty line is part of a function body and worth showing or not.

Remember how far ahead we peeked in order to avoid having to do so
repeatedly when handling multiple consecutive empty lines.

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

t/t7810-grep.sh

@@ -748,7 +748,7 @@ hello.c-#include <assert.h>
 hello.c:#include <stdio.h>
 EOF
 
-test_expect_failure 'grep -W shows no trailing empty lines' '
+test_expect_success 'grep -W shows no trailing empty lines' '
 	git grep -W stdio >actual &&
 	test_cmp expected actual
 '