Generate frequently changing API files only during ARH pull request creation

The API.comments.patch and API.md.map files would change with almost every commit, so those should not be committed as part of #5221 and related PRs. Instead, these need to be emitted whenever API Review Hub (ARH) generate a PR, so we need a hook into the system to re-run generate_api against the target (presumably HEAD) branch.

These will enable comments and source navigation in the Azure API Review extension.