grep: add option to show whole function as context

Add a new option, -W, to show the whole surrounding function of a match.

It uses the same regular expressions as -p and diff to find the beginning
of sections.

Currently it will not display comments in front of a function, but those
that are following one.  Despite this shortcoming it is already useful,
e.g. to simply see a more complete applicable context or to extract whole
functions.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7810-grep.sh

@@ -509,6 +509,20 @@ test_expect_success 'grep -p -B5' '
 	test_cmp expected actual
 '
 
+cat >expected <<EOF
+hello.c=int main(int argc, const char **argv)
+hello.c-{
+hello.c-	printf("Hello world.\n");
+hello.c:	return 0;
+hello.c-	/* char ?? */
+hello.c-}
+EOF
+
+test_expect_success 'grep -W' '
+	git grep -W return >actual &&
+	test_cmp expected actual
+'
+
 test_expect_success 'grep from a subdirectory to search wider area (1)' '
 	mkdir -p s &&
 	(