We use GitHub's APIs. We did build a version before GitHub supported it, but it required using our credentials to push directly to the main branch. That wasn't acceptable to most of our customers, so we ended up not shipping it and waiting for GitHub support.

Using the GitHub APIs means you still preserve history and commit messages they way people expect (merges through the queue and direct merges behave the same), and it lets users guarantee that the only code our mergequeue merges is the code that was in the PR.