Differ: old changes should not "swallow" incoming change when it is handled

[scofalik]

At the moment, when model.Differ handles a new incoming change, it is "transformed" by old changes. Sometimes old change "swallow" a new change. This means that the new change becomes included in the old change. For example:

// ...
} else if ( inc.offset < old.offset ) {
	old.offset = inc.offset;
	old.howMany += inc.nodesToHandle;

	inc.nodesToHandle = 0;
}

inc is included into old. But... it is inc which is then transformed by other changes, not old. After old "swallows" inc, it stays as it is, but it has conflicts with other changes. Those conflicts are not processed and differ may crash.

Instead, inc should always "swallow" old.

[scofalik]

This would crash when there are multiple changes in one model.change chain, when there are multiple inserts and removes inside one parent and then there is one big remove including previous changes.