deltas: Add a compression size heuristic for endianness detection
deltas: Add a compression size heuristic for endianness detection

I see when analyzing a delta here that due to byteswapping a negative
compression ratio of 540%, 66%, and 28%.  Let's arbitrarily pick 20%
as a threshold for detecting byetswapping.

GNOME/ostree,gatispaeglis/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,gatispaeglis/ostree,gatispaeglis/ostree,gatispaeglis/ostree,gatispaeglis/ostree,GNOME/ostree,gatispaeglis/ostree