symbolic-ref: allow refs/<whatever> in HEAD

symbolic-ref: allow refs/<whatever> in HEAD

Commit afe5d3d5 introduced a safety valve to symbolic-ref to
disallow installing an invalid HEAD. It was accompanied by
b229d18a, which changed validate_headref to require that
HEAD contain a pointer to refs/heads/ instead of just refs/.
Therefore, the safety valve also checked for refs/heads/.

As it turns out, topgit is using refs/top-bases/ in HEAD,
leading us to re-loosen (at least temporarily) the
validate_headref check made in b229d18a. This patch does the
corresponding loosening for the symbolic-ref safety valve,
so that the two are in agreement once more.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>