Fix argument parsing for --boolean and --use-git-files
Fix argument parsing for --boolean and --use-git-files

bool('0') evaluates to True, so type=bool does not work as expected.
It is necessary to use a helper function which converts strings to
True/False as expected.

Also, fix parse_boolean().

systemd/mkosi,UbiCastTeam/mkosi,UbiCastTeam/mkosi,systemd/mkosi