There was an issue with the script when passing optional rustfmt configs
without specifying a commit hash. Because these optional values are
passed via positional arguments the configs ($4) would be used in place
of the commit hash ($3). Now that we set a default value for the
optional commit hash we avoid this problem.
This new action is intended to help us maintainers determine when feature
branches cause breaking formatting changes by running rustfmt (master)
and the feature branch on various rust repositories.
Over time I expect the list of checked projects to increase.
With this action in place we can more easily test that a new feature or
bug fix doesn't introduce breaking changes. Although this action needs to
be manually triggered right now, we might consider adding it to our CI
runs in the future.
