checkout: omit "tracking" information on a detached HEAD

By definition, a detached HEAD state is tentative and there is no configured "upstream" that it always wants to integrate with. But if you detach from a branch that is behind its upstream, e.g., $ git checkout -t -b main origin/main $ git checkout main $ git reset --hard HEAD^ $ git checkout --detach main you'd see "you are behind your upstream origin/main". This does not happen when you replace the last step in the above with any of these $ git checkout HEAD^0 $ git checkout --detach HEAD $ git checkout --detach origin/main Before 32669671 (checkout: introduce --detach synonym for "git checkout foo^{commit}", 2011-02-08) introduced the "--detach" option, the rule to decide if we show the tracking information used to be: If --quiet is not given, and if the given branch name is a real local branch (i.e. the one we can compute the file path under .git/, like 'refs/heads/master' or "HEAD" which stand for the name of the current branch", then give the tracking information. to exclude things like "git checkout master^0" (which was the official way to detach HEAD at the commit before that commit) and "git checkout origin/master^0" from showing tracking information, but still do show the tracking information for the current branch for "git checkout HEAD". The introduction of an explicit option "--detach" broke this subtley. The new rule should have been If --quiet is given, do not bother with tracking info. If --detach is given, do not bother with tracking info. Otherwise, if we know that the branch name given is a real local branch, or if we were given "HEAD" and "HEAD" is not detached, then attempt to show the tracking info. but it allowed "git checkout --detach master" to also show the tracking info by mistake. Let's tighten the rule to fix this. Reported-by: mirth hickford <mirth.hickford@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>