When rendering review UIs into diffs, we'd end up creating instances for

both the original and modified files, even if we had a UI capable of

rendering a diff. This change makes it so we look up the matching Review

UI classes first and check if it's diff capable. If so, we only create

it once and use it to render the diff. If we have some other case (such

as only having one side or the other, or having a file type mismatch),

we'll instantiate each review UI and render the column.

Looked at a diff that had attachments for binary filediffs. Saw that

review UIs were instantiated and rendered correctly.