While playing around with interdiffs on review requests that have commit

histories, I encountered a crash when navigating from an interdiff back

to a single diff. The cause of this is that the diff context payload had

a commit_history_diff of null, and we therefore weren't resetting

the history diff information in the model. The commit list would then

still think we were looking at an interdiff and try to index into a

commit that didn't exist.

Navigated around through a bunch of interdiffs and normal diffs. Saw

that things no longer crashed.