grep: pass -I (ignore binary) down to external grep

We forgot to pass this option to the external grep process.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-grep.c

diff --git a/builtin-grep.c b/builtin-grep.c
index 3a51662..d3cc75e 100644
--- a/builtin-grep.c
+++ b/builtin-grep.c
@@ -289,6 +289,8 @@
 		push_arg("-E");
 	if (opt->regflags & REG_ICASE)
 		push_arg("-i");
+	if (opt->binary == GREP_BINARY_NOMATCH)
+		push_arg("-I");
 	if (opt->word_regexp)
 		push_arg("-w");
 	if (opt->name_only)