archival: don't set gzip filename header when there's no filename This mainly affects hgweb, which can generate tar.gz archives without filenames. Without this change, the header would be set to ".gz", which can confuse Safari into extracting the file and renaming it to "gz" when "Open 'safe' files after downloading" is enabled. file(1) before: hg-crew-5e51254ad4d4.tar.gz: gzip compressed data, was ".gz", last modified: Thu Dec 2 11:46:20 2010, max compression after: hg-crew-5e51254ad4d4.tar.gz: gzip compressed data, last modified: Thu Dec 2 11:46:20 2010, max compression