delete_ref(): handle special case more explicitly

delete_ref(): handle special case more explicitly

delete_ref() uses a different convention for its old_sha1 parameter
than, say, ref_transaction_delete(): NULL_SHA1 means not to check the
old value. Make this fact a little bit clearer in the code by handling
it in explicit, commented code rather than burying it in a conditional
expression.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>