Improve Git repository rebuild
- support tags: CVS tags are exported to the Git mirror and should have the precedence over branches when collecting available channels. Tags are now exported correctly from the Git repos.
- limit exported versions: we can now limit the versions that will be exported and also those that will only be exported via the "compat" channel. This works via a flag to the proc.
- hardcode openacs.org in the documentation link fo the exported repo, as the assumption is that the link will be served from the xowiki instance there
- include a download button, as per the openacs.org version in the extracted repo