github-merge-pr: add Link: tag and Committer SoB

github-merge-pr: add Link: tag and Committer SoB

If we can rebase the Pull Request, append the Link: tag and Committer SoB
tag to the end of the commit message to better reference the source of
the commit and who Approved/Committed.

In theory any modification to the commit should always have a SoB hence
a committer rebasing the Pull Request should add the SoB tag.

Signed-off-by: Christian Marangi <[email protected]>