Fix FileNotFoundError when doing out-of-source build.

If we have a project "project", decide to build it in the
"build" subdirectory, and then run:

$ coveralls -r .. -b .

We'll end up with "FileNotFoundError" since the current directory
is "build" and "filepath" (relative) points to (for instance)
"src/file.cc". Using the absolute path prevents that error.