refs: lock symref that is to be deleted, not its target

refs: lock symref that is to be deleted, not its target

When delete_ref is called on a symref then it locks its target and then
either deletes the target or the symref, depending on whether the flag
REF_NODEREF was set in the parameter delopt.

Instead, simply pass the flag to lock_ref_sha1_basic, which will then
either lock the target or the symref, and delete the locked ref.

This reimplements part of eca35a25 (Fix git branch -m for symrefs.).

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>