Add reviewers to merge commit

Description

Our external systems and our humans need to see who approved merges. We use a flow in which the approver does not always perform the merge themselves, and we may have multiple reviewers. We would like to be able to, using Git alone, determine who approved (gave thumbs-up for) a merge.

Proposal

Add an option to add a Reviewed-by: Name <email@example.com> part to a merge commit.

Links / references

Linux does this: https://github.com/torvalds/linux/commit/75599d365367fe4a5bf7b10451f19a99369af219

Mozilla also adds a reviewer to their commits: https://hg.mozilla.org/mozilla-central/rev/7d9ecc413a34 (the r= part)