git archive only works with the git protocol (ssh) and the usage of deploy keys has been nacked to avoid suprising hidden deployment issues just for one service.
This means we need to change populate_documentation.sh to download the gitlab-provided tarballs instead of git archive. It will slow down the doc generation for users of update_xml, but there seems to be no alternative.
The tarballs generation supports a path parameter (i.e. path=doc) which only extracts the required directory.