This way it's more likely that we can do a cheap rename instead of a
copy in SfxMedium::Commit().

Change-Id: I45c80cd19c3ab3bc70ecbf9793dbe1bb55994ee9
Reviewed-on: https://gerrit.libreoffice.org/47611Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>