coverage diff in pull requests #1

To discuss next year.

One aspect of the overall "reskin coverage reports" project is to add "coverage diffs in pull requests".

The script in this repo https://github.com/cppalliance/ci-automation/blob/master/scripts/gcov-compare.py compares two gcovr summary json files such as these example files https://github.com/cppalliance/ci-automation/blob/master/examples/summary.1.json and outputs the coverage diff as a plain text file https://634.url.prtest2.cppalliance.org/gcovr/coverage_diff.txt

To be done: add html and javascript such that the diff is shown as a colorful graphical chart instead of merely numbers.

The new script could be added here in gcov-compare.py, in the output section. Instead of only generating numbers, as currently, it will print the data embedded in a formatted web page with other features.

cc @vinniefalco @gregnewman