init: reject unknown args

If you pass args to `repo init` when first creating a checkout, the
repo launcher throws an error.  But the init subcommand that runs in
an existing checkout silently ignores them.  Throw a proper error.

Change-Id: I433bfcc73902d25f6b6a2974e77f6a977a75ed16
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/279696
Reviewed-by: Jonathan Nieder <jrn@google.com>
Tested-by: Mike Frysinger <vapier@google.com>