I've seen commit messages that refer to a specific issue / pull request. What is the syntax to include a specific repository's issue or pull request?
Use the documented auto-linking format for issues across repositories.
{owner}/{repository}#{issue_number}
mojombo/jekyll#1
When such formatted text is present in a commit message, it's automatically transformed into a clickable link which will redirect one to https://github.com/{owner}/{repository}/issues/{issue_number}