When grabbing the raw diff of a DiffSet on a multi-commit review

request, the diff would contain the cumulative diff and the diffs of all

files across all commits. This was due to the code attempting to gather

all FileDiffs pointing to the DiffSet, which included those from

commits.
This change updates the logic to be smarter, inspecting what we're

working with and fetching the files based on the capabilities of the

parent object being passed in.

Unit tests passed.
Manually testing that I was only getting the cumulative diff when using

the diffs API or when using the "Download Diff" link.