Let git-checkout always drop any detached head

Let git-checkout always drop any detached head

We used to refuse leaving a detached HEAD when it wasn't matching an
existing ref so not to lose any commit that might have been performed
while not on any branch (unless -f was provided).

But this protection was completely bogus since it was still possible
to move to HEAD^ while still remaining detached but losing the last
commit anyway if there was one.

Now that we have a proper reflog for HEAD it is best to simply remove
that bogus (and admitedly annoying) protection and simply display the
last HEAD position instead.  If one wants to recover a lost detached
state then it can be retrieved from the HEAD reflog.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>