checkout: suppress tracking message with "-q"

checkout: suppress tracking message with "-q"

Like the "switched to..." message (which is already
suppressed by "-q"), this message is purely informational.
Let's silence it if the user asked us to be quiet.

This patch is slightly more than a one-liner, because we
have to teach create_branch to propagate the flag all the
way down to install_branch_config.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>