When run with --diff, fmt.sh now only checks for the .java files that have been
changed since the fork point. This makes local git pre-push hooks more quiet
while preserving the safe CI behavior (which doesn't use --diff).

changelog_begin
changelog_end

Pull Request Checklist

• Read and understand the contribution guidelines
• Include appropriate tests
• Set a descriptive title and thorough description
• Add a reference to the issue this PR will solve, if appropriate
• Include changelog additions in one or more commit message bodies between the CHANGELOG_BEGIN and CHANGELOG_END tags
• Normal production system change, include purpose of change in description

NOTE: CI is not automatically run on non-members pull-requests for security
reasons. The reviewer will have to comment with /AzurePipelines run to
trigger the build.