largefiles: fix download of largefiles from an empty list of changesets The empty list was interpreted as all revisions - just like None is. The empty list is now handled explicitly.

largefiles: fix off-by-one error on pull --all-largefiles Test output is changed in a case where one revision was pulled, but because of the off-by-one error it thought that 0 revisions were pulled ... and because of another bug it thus (tried to) fetch largefiles for all revisions. After this change it no longer reports failure when it failed while trying to fetch largefiles it shouldn't fetch. Largefiles that it shouldn't fetch but managed to fetch anyway will now correctly be missing later on. This change thus resolves some of unexplained test output introduced in 1e4eb1faba6e.