Filter paths consistently for executed and unexecuted files.

Previously we only passed the basename to the exclusion path
checker, which, if that was checking a pathname component, wouldn't
work for unexecuted files. Pass the absolute path like we do when
reading the .gcov file.

Fixes eddyxu/cpp-coveralls#19 .
Merge pull request #20 from smspillaz/issues/19

Filter paths consistently for executed and unexecuted files.