push: allow delete single-level ref

push: allow delete single-level ref

We discourage the creation/update of single-level refs
because some upper-layer applications only work in specified
reference namespaces, such as "refs/heads/*" or "refs/tags/*",
these single-level refnames may not be recognized. However,
we still hope users can delete them which have been created
by mistake.

Therefore, when updating branches on the server with
"git receive-pack", by checking whether it is a branch deletion
operation, it will determine whether to allow the update of
a single-level refs. This avoids creating/updating such
single-level refs, but allows them to be deleted.

On the client side, "git push" also does not properly fill in
the old-oid of single-level refs, which causes the server-side
"git receive-pack" to think that the ref's old-oid has changed
when deleting single-level refs, this causes the push to be
rejected. So the solution is to fix the client to be able to
delete single-level refs by properly filling old-oid.

Signed-off-by: ZheNing Hu <adlternative@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>