symbolic-ref -d: do not allow removal of HEAD

symbolic-ref -d: do not allow removal of HEAD

If you delete the symbolic-ref HEAD from a repository, Git no longer
considers the repository valid, and even "git symbolic-ref HEAD
refs/heads/master" would not be able to recover from that state
(although "git init" can, but that is a sure sign that you are
talking about a "broken" repository).

In the spirit similar to afe5d3d5 ("symbolic ref: refuse non-ref
targets in HEAD", 2009-01-29), forbid removal of HEAD to avoid
corrupting a repository.

Signed-off-by: Junio C Hamano <gitster@pobox.com>