Up to now, deltas needed to be enterily in the packfile, and we tried
to decompress then in their entirety over and over again.

Adjust the logic so we read them as they come, just as we do for full
objects. This also allows us to simplify the logic and have less
nested code. The delta resolving phase still needs to decompress the
whole object into memory, as there is not yet any streaming
delta-apply support, but it helps in speeding up the downloading
process and reduces the amount of memory allocations we need to do.