Allow / as root folder

When / is the only common directory of the coverage report, the report generation fails.
The root dir "/" is trimmed at several functions, what leads to a path like "home/somefile.php" which is of course not found and thus not imported to the coverage report (at least not with coverage-html).

This PR fixes this issue and allows coverage reports with "/" as the only common path of the report.
I hope the tests point out the problem.