Files incorrectly marked as not anchored in iterative patch reviews

HTR:

1. create a patch review with two iterations
2. move the revision slider, note the 'crossed anchor' icon popping up when you move the from revision
3. add a reviewer, start the review
4. as the reviewer complete the review
5. add another patch revision
6. as the reviewer visit the review
7. note that the 'lock' icon is now displayed for the files in the patch that you've previously reviewed, and that the content shown is incorrect

Both are these are caused by incorrectly determining whether the patch is anchored (on page-load time, and on slider-move) - the check is done on the subsequent revisions, which aren't anchored in our model. Getting that check wrong causes the icons to be wrong, but also if the check is done at load time (as it is when revisiting a previously reviewed revision), it causes the 'lock' icon to be shown and the content of the files to not be rendered properly.