remotefilelog: bump pack file formats to version 2 Drop support for format 0 and 1 packs. Version 2 will be different in that it'll use zlib compression instead of lz4, as the latter isn't really suitable for core. If I can make it work, I'll make compression engines pluggable, but the logic for handling these files is spread out enough it's a little tricky to get that right. Test changes are only due to pack files being renamed. The contents of the packs are the same. Differential Revision: https://phab.mercurial-scm.org/D4943