GitHub mentions are good—in the pull request thread you open on GitHub. In commit messages, not so much. The same rule applies for issue references; URLs should

GitHub mentions are good—in the pull request thread you open on GitHub. In commit messages, not so much. The same rule applies for issue references; URLs should be preferred (or at least something like GH-XXXX) to simple hashtag numbers in commit messages, the improvement matters a lot when it does, but you can only get it if you do it from the beginning (Git commits being immutable).