patman: force git log commands to not use color

Colored logs confuse patman when analyzing logs.
Add --no-color option in git log commands in case
the default config has color.

Signed-off-by: Albert ARIBAUD <albert.u.boot@aribaud.net>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@ti.com>