AppUpdater: Add ability to include metadata files in diffs

Branched from https://github.com/endlessm/xdelta3-dir-patcher/pull/11 so don't merge until that PR is included.

[endlessm/eos-shell#3985]
@sgnn7 This is good enough for our usage, but I think it would be worth looking at an interface for adding an arbitrary file to the xdelta. There's no reason this needs to be a .info file in the root.