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.

Testing done:
Navigated around through a bunch of interdiffs and normal diffs. Saw
that things no longer crashed.

Reviewed at https://reviews.reviewboard.org/r/10309/