Remove coverageGutters, make coverage relative to repo root, add repo root

[nohwnd]

PR Summary

Add detection of repository root, to find a common path for reports and potentially other stuff. Simply look for .git folder, which is a common way we do it in other repos that works well, and .git is basically used everywhere. If not found use the working directory, which is most often the repo root anyway. User can override the value if it does not work for them. It allows CI tools to work well, because they usually care about relative paths to the checkout directory, without forcing all paths to absolute.

Remove CoverageGutters format from coverage, because it was just JaCoCo with the paths correctly formed against coverage.xml location, which was in the repo root.

Removes the "get common folder" function which returns result dependent on the files you provided to the coverage in non-obvious way.

Fix #2668

PR Checklist

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[nohwnd]

There are 2 new options, run.repoRoot, and coverage.reportRoot, not sure if making the names similar is genius or stupid. But I like both names.

[nohwnd]

finally green in case you want to have a look @fflaten