The current layout uses a prefix of two bytes as the initial dir
and a second directory inside that with the superblock. This
updates the list code to handle that.
https://bugzilla.gnome.org/show_bug.cgi?id=721799
This has a very basic level of functionality (deltas can be generated,
and applied offline). There is only some stubbed out pull code to
fetch them via HTTP.
But, better to commit this now and improve it from a known starting
point, rather than have it languish in a branch.