mx archive: only print overwrite warning on files

--HG--
extra : histedit_source : 7b1b981d3474dd53864abcfe29df9071baa094a7
mx clean: delete distributions

--HG--
extra : amend_source : b9d5d75de4090a73e6c7f4ce7b335888ee3537a3
extra : histedit_source : 2fa83bc85a3d31cbc74a3707a98b332b6057f392