This command is basically a fork of `cli/scores`, except it outputs the
format described in #1773. I started by copying cli/scores and
sharness/test_cli_scores.t, and made appropraite modifications.
You can check out the example-github-output.json to get a feel for the
new format. I also added a compat header in `analysis/output.js`, and
made the necessary adjustments to the CLI harness.
Test plan: The sharness test runs the real command and saves output in
its success case, looking at that JSON is sufficient. I also manually
ran it on the @sourcecred project.
