update-ref: use {old,new}-oid instead of {old,new}value

update-ref: use {old,new}-oid instead of {old,new}value

The `git-update-ref` command is used to modify references. The usage of
{old,new}value in the documentation refers to the OIDs. This is fine
since the command only works with regular references which hold OIDs.
But if the command is updated to support symrefs, we'd also be dealing
with {old,new}-refs.

To improve clarity around what exactly {old,new}value mean, let's rename
it to {old,new}-oid.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Acked-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>