Strange dmd -cov bug

["Luís]

The coverage is quite broken. The paths are broken 
(http://d.puremagic.com/issues/show_bug.cgi?id=11214), and I 
found a bug where if you activate the merge flag (in cover.d) it 
will incorrectly sum the coverages from the previous run. I had a 
fix for that, but it seems my stash of it got lost. When I find 
the time I'll recreate it and do a pull request (I just submitted 
3 bug reports, it takes time to reduce the test cases, etc.)