tagged-git: use fetch --tags to overwrite local tags.

[attila-lendvai]

without --tags git will not overwrite the local tags if someone has moved them in the remote with a push --force.

this is a step towards making tagged-git work seamlessly in a setup where project authors can move a tag called 'quicklisp' to the revision they propose to go into quicklisp.

unfortunately i couldn't test any of this.