TestRunner: Added --chart to display a chart of performance tests

This uses the Chart.js javascript library. Everything is put into a standalone .html file and opened with the default browser. I also simplified the text output to use the same data as the chart. You can now use a commit range with git's ".." syntax. The --ci option will use results from CI (you'll need to fetch them first):

  $ git fetch https://gitlab.haskell.org/ghc/ghc-performance-notes.git refs/notes/perf:refs/notes/ci/perf
  $ python3 testsuite/driver/perf_notes.py --ci --chart --test-env x86_64-darwin --test-name T9630 master~500..master

Summary of changes:

• Refactoring - extract into a top level function:
  • get_commit_message (memoized)
  • commit_log
  • get_commit_metric (memoized)
• Add new cli parameters (--chart, --ci, ...)
• Output of the script is now either
• Add code to generate chart html file
• Remove old code to aggregate and print (to stdout) the metrics.
  • This is slightly simpler
  • The data matches that visualized by the chart.