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

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 <redacted>
Signed-off-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>