checkout: omit "tracking" information on a detached HEAD

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>