See comment in !151. Using the "--initial" option of lcov we collect
the coverage of all compiled files and merge them later into the final
report. This way we can see which files are built but never executed
by the test suite.

Because the --initial switch also collects files in the ccache directory
we have to point it to the build directory instead, which in turn breaks
--no-external. Instead of using --no-external in the collection step,
filter out any files not in the source tree in the final coverage job
through a path filter.