Commit message diff is a fake FileDiff. These are created during the push and should be displayed as a first FileDiff in the list. If MozReview knows that a commit message filediff is present it is using JS and CSS to rename its unique filename to "commit-message". For this URL [1] no commit message FileDiff is returned by the server, but front-end has received an information that a commit message FileDiff will be displayed and renamed the "toolkit/components/Extension.jsm" to "commit-message". [1] https://reviewboard.mozilla.org/r/117362/diff/1-2/