[js2]

Just to make sure we're not talking past each other: OP wanted to test both "GitHub (and git)". OP could have tested the git portion locally.

But to engage you about the GitHub part: I believe that under the covers, GitHub is still using something substantially similar to git as the repo storage format. Git has no inherit limitations on number of commits. Eventually you run out of disk space, and possibly memory and/or CPU during repacking. You could turn off GC and let the repo remain unpacked. You might eventually run out of inodes. During cloning (and pulling), git implicitly creates pack files, so a clone/pull will also take a long time (CPU and or memory again) on an unpacked repo. This is why git periodically repacks.

If I had to guess, GitHub also has no inherit limits. Creating commits was probably periodically repacking on the git backend, consuming increasing amounts of resources.

I would be surprised if the GitHub API (the Ruby on Rails code) takes much resources at all.

Creating endless PRs is something you can simulate locally with two copies of a repo. You can use "git ls-remote" against a GitHub-hosted repo with PRs in it to see how it exposes PRs as references that are not normally cloned.

Regardless, I think that OP could and should have satisfied their curiosity about how git works locally, especially with respect to whether it has an inherent limits. And they could have satisfied their request about GitHub resource limits with a support request.