When paginating in the diff viewer, we were inadvertently resetting the

commit ranges. This was due to merely specifying the new page to load

from without passing in the existing ranges that were selected, causing

those to reset.
Now, when handling a page selection, we fetch any existing commit range

information and pass it to _navigate().
Unit tests were added to ensure this doesn't regress.

Unit tests passed.
Manually tested with a diff containing multiple pages and commits.