Aside from avoiding a tiny bit of work, this makes it transparently
obvious that old_sha1 and new_sha1 are identical. It is arguably a
bit silly to have to set new_sha1 in order to verify old_sha1, but
that is a problem for another day.
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>