short circuit if there is no change between source and destination branch

[abbiejones]

There is a bug in the "merge" Github Action causing it to fail when there is no difference between the source branch and destination branch. See this GHA output for reference: https://github.com/OpenSesame/Catalog/actions/runs/24209438648/job/72192345956?pr=17939

{"base":"develop","head":"OpenSesame:refs/heads/release/0000","maintainer_can_modify":true,"title":"Automated Merge"}
< HTTP 422
{"message":"Validation Failed","errors":[{"resource":"PullRequest","code":"custom","message":"No commits between develop and release/0000"}],"documentation_url":"https://docs.github.com/rest/pulls/pulls#create-a-pull-request","status":"422"}
Error creating pull request: Unprocessable Entity (HTTP 422)
No commits between develop and release/0000
Failed to get PR URL for merge of refs/heads/release/0000 into develop
One or more merges failed:
 - develop

Because there are no commits between the source and destination, it fails to create a PR to merge downstream changes.

This PR fixes that issue by short circuiting when source and destination are the same.

[Copilot]

Pull request overview

Fixes the merge GitHub Action failing when attempting to open a PR between identical source/destination branches by short-circuiting no-op merges before PR creation.

Changes:

• Derive the source branch name from GITHUB_REF within open_and_merge_pull_request.
• Add an “ahead commits” check to skip PR creation when the source branch has no commits ahead of the destination branch.

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