pack: enhance docstring on deltify_pack_objects

Change-Id: If208d523b3d7bb4d83f7ded2fec47d5c1052bbf0
pack.create_delta: move encode_size to not be a nested function

No functional change, just tidies things up a little bit.

Change-Id: I48810dec9788584812dabe36976de60d24fce498
pack.create_delta: rename a couple of variables to make the code clearer

Change-Id: I5fed216800445277d6e069e0b20051006330d405
pack.create_delta: correctly encode copy operations up to 2^24 in size

At present we still produce incorrect results for operations larger
than this size. That defect will be fixed in a follow-up to this
change.

Change-Id: Id63b500ae514caa4339063cde2d6486309a8a056