Fix instance obj refresh()

The refresh() method pulls a fresh copy of the instance but if any
fields changed when doing so, they are left marked as changes that
are not yet committed.

Also clarified the doc string for obj_what_changed() to state that a set
is returned, not a list.

This method is not used yet, so I didn't file a bug for this.

Change-Id: I83fbce07936fea2bc936441c5c6594f65da443a7
Merge "Fix instance obj refresh()"