Update comparing-commits.md to mention "OWNER:REPO:BRANCH" new pattern (github#34927)

Author: muzimuzhi

content/pull-requests/committing-changes-to-your-project/viewing-and-comparing-commits/comparing-commits.md

@@ -58,6 +58,8 @@ You can compare your base repository and any forked repository. This is the view
 
 To compare branches on different repositories, preface the branch names with user names. For example, by specifying `octocat:main` for `base` and `octo-org:main` for `compare`, you can compare the `main` branch of the repositories respectively owned by `octocat` and `octo-org`.
 
+You can also preface the branch name with a user name and a repository name. For example, specifying `octocat:awesome-app:main` would use the `main` branch in the `octocat/awesome-app` repository. This can be useful in large organizations, where you might have an upstream repository and a fork both owned by the organization. For more information, see "[AUTOTITLE](/pull-requests/collaborating-with-pull-requests/working-with-forks/about-forks)."
+
 Here's an example of a [comparison between two repositories](https://github.com/github-linguist/linguist/compare/master...octocat:master).
 
 ## Comparisons across commits