I remember listing out 4-5 of these in the past when the topic has come up, but the only old one I can remember/find right now is https://github.com/dspinellis/git-issue
Edit - Found some more using duckduckgo instead of google:
* https://github.com/jashmenn/ditz (last update 12 years ago)
* https://bugseverywhere.org/ (I think this was the most popular at one point)
* https://github.com/marekjm/issue (this is separate from git-issue above)
* A blog post from 2012 that lists these and a few others (half the links are dead): http://www.cs.unb.ca/~bremner/blog/posts/git-issue-trackers/
From the blog post, the additional ones where the links still work:
* https://github.com/chilts/cil (last updated 11 years ago)
* http://syncwith.us/sd/ (last updated 6 years ago)