Capture commit hash in submission description

[zanerock]

Overview

Because you can push updates to a branch after creating a PR, the PR description, which caries the certifications, needs to capture the git version it was created against with instructions to only merge if the last commit hash matches.

Deliverables

• Add the following to the submit description:

## Merge checks

* [ ] Verify the final commit hash matches: <first 7 commit hash>
* [ ] Verify that all policy certifications have been made or exceptions duly explained.