dev-cmd/update-maintainers: If there are changes, auto-update man pages

- I noticed this in a PR recently, maybe the maintainer didn't think
  they would have to run `brew man` for this? In that case there's an
  extra step after the PR is raised, red CI checks on the PR (which is
  demoralising) and potentially requires a reviewer to point this out.
- This runs `brew man` if the `README` file has a diff as part of this
  command. Then the user has to `git add .` and their PR is (probably)
  good.