TIL these are a thing. Thanks! (Just a regular joe on the internet, not the author)

It might solve it for git, but this looks like something the Review Board team came up with, and they have to integrate with many other version control systems like SVN, CVS, Perforce..etc. Seems like this is meant to address supporting many different version control systems with a single format.

I've worked at a place that used Review Board, and SVN as their primary vcs, but many devs used a local git-svn mirror for their work. Sometimes this caused problems with uploading diffs, especially if svn and git-svn were being mixed in one review. Having the Review Board cli generate a common diff format for both would have helped with that.