Well, APFS is much better suited to this kind of workflow. Create a space shared logical volume inside your container and turn spotlight off on that particular volume (and if you’d like, make that volume case sensitive ). There’s no need to separate that out on a diskimage
There's still the problem of Time Machine (or any other backup software you use) needing to do a complete deep scan of the volume to ensure you haven't made any changes. If you know a git repo is effectively in a "read-only" state—something you just keep around for reference, or an old project you might "get back to" a few years from now—it can speed up those backups dramatically to put the repo into some kind of archive. Disk images, for this use-case, are just archives you can mount :)
