Can you key off of [the index file](https://github.com/geany/geany-themes/blob/master/index/index.json)? Whenever what most people would call a "release" is made, this JSON file is updated. It's used on [Geany's website](https://geany.org/download/themes/) to keep up-to-date.
Alternatively I can try and make a Git tag for each "release" then you can go from that as a release version, instead of having to update on each individual commit.
Let me know, I can adjust things a bit if needed.