Rather than having a utility method that creates a new ChangeUpdate on
invocation, get the update from the ChangeContext. Also split out the
post-update steps into postUpdate.
The end result is admittedly a little convoluted, requiring assisted
injection just to create the op to pass to the BatchUpdate. (It should
still be a little shorter than the original.) However, we don't expect
most new Ops to require assisted injection, since they can typically
get everything they need from the context objects.
Change-Id: I4d6024dd00b00bbc1825e700b1eda0923f29d278
bce0e922c4