libsvn_fs_fs/structure says that a pack file starts with an svndiff-encoded integer. Where is that integer written, please?
serialize_revprops_header() doesn't write an svndiff-encoded integer, so I suspect the two binary bytes at the start of a revprops pack file are actually an artifact of zlib level-0 compression, rather than an svndiff integer written by svn's code. Is that correct?
