grep: use memmem() for fixed string search

Allow searching beyond NUL characters by using memmem() instead of
strstr().

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

t/t7008-grep-binary.sh

@@ -51,4 +51,8 @@ test_expect_success 'git grep -q ina a' '
 	test_cmp expect actual
 '
 
+test_expect_success 'git grep -F ile a' '
+	git grep -F ile a
+'
+
 test_done