config: Don't fill in the parser with all defaults

config: Don't fill in the parser with all defaults

There's no need to fill the parser with all the built in defaults since
we can simply copy them to the config from the class dict. This allows
us to first set the values from the old {git,gbp}-* commands and then
only overwrite changed values from the newer "gbp <command>" versions.

Otherwise we'd overwite all old style config with the new style one.
This also fixes the problem where the default section wouldn't be read
at all if the command doesn't even have an empty section in any gbp.conf
file.