checkout: tone down the "forked status" diagnostic messages

checkout: tone down the "forked status" diagnostic messages

When checking out a branch that is behind or forked from a
branch you are building on top of, we used to show full
left-right log but if you already _know_ you have long history
since you forked, it is a bit too much.

This tones down the message quite a bit, by only showing the
number of commits each side has since they diverged.  Also the
message is not shown at all under --quiet.

Signed-off-by: Junio C Hamano <gitster@pobox.com>