Fix git branch -m for symrefs.

Fix git branch -m for symrefs.

This had two problems with symrefs. First, it copied the actual sha1
instead of the "pointer", second it failed to remove the old ref after a
successful rename.

Given that till now delete_ref() always dereferenced symrefs, a new
parameters has been introduced to delete_ref() to allow deleting refs
without a dereference.

Signed-off-by: Miklos Vajna <redacted>
Signed-off-by: Junio C Hamano <redacted>