WIP: Fix paths and numbering in generated line directives
The first two commits deal specifically with coverage by emitting full (relative) paths. Note that paths are not explicitly relative but instead --directory is not canonicalized. In practise this should be no problem: The directory is taken relative to the executing processes cwd anyway but alternatively, resolving
csource_filename relative to
CodeContext.directory might be preferable.
The last commit attempts to fix jumping between vala and c files. A detailed description is given there.
Well, this is WIP and I'm glad for any suggestions.