The config file code needs to keep track of the actual
`git_config_file` structure, as it not only contains the path
of the current configuration file, but it also keeps tracks of
all includes of that file. Right now, we keep track of that
structure via the `git_config_parser`, but as that's supposed to
be a backend generic implementation of configuration parsing it's
a layering violation to have it in there.

Switch over the config file backend to use its own config file
structure that's embedded in the backend parse data. This allows
us to switch over the generic config parser to avoid using the
`git_config_file` structure.