Default to extra repo for new packages when no repo is specified

Since the 1.1 release the --repo flag is only allowed for new packages, while the -t and -s flags are only allowed for existing packages.

This breaks release scripting in case one needs to release a set of packages which includes both new and existing packages (notably the kde-build scripts).

Default to extra for new packages to fix this issue.

If extra is not the intended target of a new package and one forgets to pass the --repo flag, the package can be easily moved afterwards.