Please see bug 1345498 comment 22. The commit https://hg.mozilla.org/integration/autoland/rev/b17b5fdb4ae7 landed in that bug contains a file which is a commit message.

It seems to me that there are various inconsistent in the diff viewer of MozReview for commit message. In https://reviewboard.mozilla.org/r/118206/diff/5/ if you compare orig..2, you would see both "commit-message" and a "commit-message-800ba", while orig..1 has only "commit-message". It indicates that "commit-message-800ba" is something introduced from 1..2, however, 1..2 only has change to "commit-message" as well. In 4..5, you can see three commit-message files in the list, which are "commit-message", "commit-message-800ba", and "commit-message-875d6". This may be related.

I filed this earlier as bug 1346321 (and observed comment 1 there as well). Sorry for duplicated work. Marking as a dupe.

Renaming first file to "commit-message" happens in the front-end only. 1-2 had no change in commit-message and the filediff shouldn't be renamed (see bug 1345740).