get_maintainer.pl: Add --git to look up CCed in git history

Add the --git parameter, else recent contributors are left out of the CC list.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>