git-checkout: safety check for detached HEAD checks existing refs

git-checkout: safety check for detached HEAD checks existing refs

Checking for reachability from refs does not help much if the
state we are currently on is somewhere in the middle.  We will
lose where we were.

So this makes sureh that HEAD is something directly pointed at
by one of the existing refs (most likely a tag for a user who
has been "sightseeing").

Signed-off-by: Junio C Hamano <junkio@cox.net>