Add workflow which generates a diff of the generated DD XML file

[maarten-ic]

No description provided.

[maarten-ic]

The diff is generated okay (link), but a workflow triggered from a fork doesn't have permission to comment on the Pull Request. This is a security feature of GitHub to prevent that anyone that can create a PR can exfiltrate a secret token with write access to the repository. That's why the Action is failing.

Some options to work around this:

1. Use the pull_request_target event trigger, with some checks to make this safe-ish: https://michaelheap.com/access-secrets-from-forks/
2. Use the workflow_run trigger to make a comment to the Pull Request after the diff workflow has run: https://docs.github.com/en/actions/writing-workflows/choosing-when-your-workflow-runs/events-that-trigger-workflows#workflow_run

I think option 2 is the better of these, what do you think @olivhoenen?

[olivhoenen]

Option 2 sounds good indeed.

[maarten-ic]

The workflow is now split into two parts:

1. pr_diff.yml is triggered by the pull_request event. This creates a diff of the generated DD XML (see output).
2. pr_diff_comment_on_pr.yml is triggered by the workflow_run event, and will run when workflow 1 is successful. This workflow adds a comment to the PR so reviewers can easily navigate to the diff.

N.B. the workflow_run event only triggers for workflows defined in the default branch (develop). Therefore no comment is generated on this PR, because the second workflow is not yet committed to develop.
Both workflows are tested successfully in my fork, see maarten-ic#2.

[SimonPinches]

LGTM