compat/test_pack: add test that proves we correctly handle medium-sized objects

What I mean by "medium" in this case is that they're big enough to
require three bytes worth of length information in a copy operation,
but not big enough to require decomposing a copy operation into
multiple sequential copy operations.

Change-Id: I82c28bf8bae5c5c9ed06b79f7380bcdcf4c501e3
Update NEWS.
compat/test_pack: add test that proves large copies in a delta encode correctly

Change-Id: I1a6104344cd960c62273539546cf1581510707f3
Disable test_delta_large_object for the moment, since it is slow.