Only considers PRs that have been merged it listed on a commit

[ForestEckhardt]

There is a possibility that a non-merged PR can be listed on a commit this will cause this action to fail if that non-merged PR does not have an image label. This makes it so that still open PRs are not considered.

Checklist

• [x] I have viewed, signed, and submitted the Contributor License Agreement.
• [ ] I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• [ ] I have added an integration test, if necessary.
• [x] I have reviewed the styleguide for guidance on my code quality.
• [x] I'm happy with the commit history on this PR (I have rebased/squashed as needed).

[robdimsdale]

@ForestEckhardt LGTM. Merge when you want.