Whenever beta and release candidates are tagged in git, we don't want that to lead to the generation of especial tagged packages under master, but normally dated ones.

As far as it's something decided 100% automatically by the low-level git-package utility, this is about:

1) Add a new option to git-package, in order to be able to avoid the automatism and force dated builds.
2) Fix the higher level moodle-package so, for master it passes that option.