Run rsync command-line through a shell, so that single-quotes around excludes...

Run rsync command-line through a shell, so that single-quotes around excludes are interpreted (Closes: #677410)

When --exclude 'something' is passed directly to rsync without going through
a shell, rsync tries to match a file whose name contains single quotes, which is
definitely not what we want it to do: exclude arguments protected with single
quotes are meant to be passed to a shell.