git-plugin has extensions to support vary requirements by downstream projects such as github-branch-source-plugin and bitbucket-branch-source-plugin, but lack support for custom changelog computing. Let's make GitSCM.computeChangeLog extensible.

There're lots of use-cases like JENKINS-47870 JENKINS-51633 JENKINS-27080, and cases JENKINS-48687 JENKINS-44849 in downstream plugin.

Refer to the changelog GitHub label on the git plugin for pull request examples that are proposing modifications to changelog related capabilities.