In a GitHub issue, you can reference a commit and issue by simply adding them in the message like so:
Commit e103934519bfbd4b855d678cce1b7f0bac3dc548 closes #84
This will hyperlink (and autocomplete) issue #84 as well as shorten and hyperlink to the commit that matches the sha.
Additionally, and the best part, if you type
closes #84, it will close the issue number referenced. In this example, issue number 84.
This can be seen on GitHub here: https://github.com/WebApiContrib/WebAPIContrib/pull/84#issuecomment-12080311