Merge branch 'sb/checkout-explit-detach-no-advice'

Merge branch 'sb/checkout-explit-detach-no-advice'

"git checkout --detach <branch>" used to give the same advice
message as that is issued when "git checkout <tag>" (or anything
that is not a branch name) is given, but asking with "--detach" is
an explicit enough sign that the user knows what is going on.  The
advice message has been squelched in this case.

* sb/checkout-explit-detach-no-advice:
  checkout: do not mention detach advice for explicit --detach option