Index a field "editby" with a value for each account ID that has an
edit on this change. Intentionally don't expose editby:'USER' because
only change edit owners can see them.
Given that "has:edit" predicate outcome depends on the existence of
the change edit ref and not on its content, only reindex the changes
on change edit ref creation and deletion, but not on its mutation.
Bug: Issue 3291
Change-Id: I2d541c781d057cdf9dcacfc02f81591ce6872595
9952870b5e