checkout: introduce --detach synonym for "git checkout foo^{commit}"

checkout: introduce --detach synonym for "git checkout foo^{commit}"

For example, one might use this when making a temporary merge to
test that two topics work well together.

Patch by Junio, with tests from Jeff King.

[jn: with some extra checks for bogus commandline usage]

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>