checkout: make advice when reattaching the HEAD less loud

checkout: make advice when reattaching the HEAD less loud

When switching away from a detached HEAD with "git checkout", we give a
listing of the commits about to be lost, and then tell how to resurrect
them since 8e2dc6a (commit: give final warning when reattaching HEAD to
leave commits behind, 2011-02-18).

This is a good safety measure for people who are not comfortable with the
detached HEAD state, but the advice on how to keep the state you just left
was given even to those who set advice.detachedHead to false.

Keep the warning and informational commit listing, but honor the setting
of advice.detachedHead to squelch the advice.

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