loader: Fix rsync failures by retrying associated commands

Fetching CVS repository data using rsync often fails with the following error
(especially with archived repositories hosted on sourceforge):

rsync error: some files/attrs were not transferred (see previous errors) (code 23)

It seems the only way to mitigate that issue is to retry the rsync command
until it succeeds.

So add a rsync_retry decorator and apply it to a new method in the loader
wrapping the call to subprocess.run executing the rsync command.

Also use rsync option to compress file data during the transfer.