make diffPreview and messagePreview mutually exclusive

Created by: camdencheek

A few places in our backend expect message preview and diff preview to be mutually exclusive in order to detect whether a CommitMatch is type:commit or type:diff. This is not ideal, but for now, this fixes an issue with the new commit/diff search path and makes the condition explicit with a comment.

Fixes #25723 -- at least to the level that current commit/diff search works