Make the clone script retrieve all branches and tags

By default, not all tags are retrieved. So a command like "./clone.py release-5.6.0-HF31 will require a "git fetch --tags" before.

Improve the clone script to make it retrieve all branches and tags by specifying explicit refspecs such as:

git fetch origin +refs/heads/*:refs/remotes/origin/* refs/tags/*:refs/tags/*