Annotate merge commits with GitHub error annotations

[motlin]

Modifies the action to create GitHub error annotations for each merge commit found.

• Iterates through merge commits: Replaces the previous check for merge commits with a loop that iterates over each merge commit found, using git log --merges --format=%H.
• Generates error annotations: For each merge commit, echoes a GitHub error annotation in the format ::error::Merge commit found: $COMMIT_HASH, providing direct feedback in the GitHub Actions interface.
• Removes previous echo statement: The echo statement that previously printed out the merge commits has been removed, streamlining the output to focus on the error annotations.

---

For more details, open the Copilot Workspace session.