prevent HEAD reflog to be interpreted as current branch reflog

prevent HEAD reflog to be interpreted as current branch reflog

The work in progress to enable separate reflog for HEAD will make it
independent from reflog of any branch HEAD might be pointing to. In
the mean time disallow HEAD@{...} until that work is completed. Otherwise
people might get used to the current behavior which makes HEAD@{...} an
alias for <current_branch>@{...} which won't be the case later.

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