grep -O: Do not pass color sequences as filenames to pager

With a .gitconfig like this:

 [color]
	ui = auto
 [color "grep"]
	filename = magenta

if stdout is a terminal, the grep machinery will output the color
sequence \e[36m before each filename in its output.

In the case of "git grep -O foo", output is argv for the pager.
Disable color when calling the grep machinery in this case.

Signed-off-by: Nazri Ramliy <ayiehere@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/grep.c

@@ -997,6 +997,7 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 	if (show_in_pager == default_pager)
 		show_in_pager = git_pager(1);
 	if (show_in_pager) {
+		opt.color = 0;
 		opt.name_only = 1;
 		opt.null_following_name = 1;
 		opt.output_priv = &path_list;