cmd_update_ref(): make logic more straightforward
cmd_update_ref(): make logic more straightforward

Restructure the code to avoid clearing oldsha1 when oldval is unset.
It's value is not needed in that case, so this change makes it more
obvious that its initialization is consistent with its later use.

Signed-off-by: Michael Haggerty <c24c152f145c57791e809b126db9e61bd1c1782d@alum.mit.edu>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git