t5516: don't use HEAD ref for invalid ref-deletion tests

t5516: don't use HEAD ref for invalid ref-deletion tests

A few tests in t5516 want to assert that we can delete a corrupted ref
whose pointed-to object is missing. They do so by using the "main"
branch, which is also pointed to by HEAD.

This does work, but only because of a subtle assumption about the
implementation. We do not block the deletion because of the invalid ref,
but we _also_ do not notice that the deleted branch is pointed to by
HEAD. And so the safety rule of "do not allow HEAD to be deleted in a
non-bare repository" does not kick in, and the test passes.

Let's instead use a non-HEAD branch. That still tests what we care about
here (deleting a corrupt ref), but without implicitly depending on our
failure to notice that we're deleting HEAD. That will future proof the
test against that behavior changing.

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>