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