Squelch overzealous "ignoring dangling symref" in an empty repository

Squelch overzealous "ignoring dangling symref" in an empty repository

057e713 (Warn use of "origin" when remotes/origin/HEAD is dangling,
2009-02-08) tried to warn dangling refs/remotes/origin/HEAD only when
"origin" was used to refer to it.  There was one corner case a symref is
expected to be dangling and this warning is unwarranted: HEAD in an empty
repository.

This squelches the warning for this special case.

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