Update bin/ursula to check for missing git commits.

To prevent inadvertently running ursula with out-of-date
ansible or config trees, fetch remote `origin`, and die if
`origin/master` has commits missing from `HEAD` in either tree.

This behavior can be disabled by passing argument `--force/-f`.

This feature currently assumes the existence of an remote named `origin`
in both trees, and will die if no such remote is present.

Additionally, add a `--help` flag to ursula.