Fixed a bug that would cause combined reports to always be 100% covered

There is a bug in how diff-cover calculates coverage when given multiple coverage XML files: If one XML report contains "file.py" and the other file does not, then the current implementation will report 0 violations for "file.py".

This means everything is reported as 100% covered all the time :(

@sarina Please review.  I'd like to get this merged early Tuesday, as diff coverage is broken right now that we've switched to combined reports.