fix pack.sh test failure from reuse-deltas patch by tweaking progress output

reuse existing deltas when creating pack files tested by thomas, naddy, and myself

compress delta data from delta_cache directly into pack file

set a cap on the amount of memory we use to store encoded deltas

encode short deltas in memory instead of writing them to a temporary file

map raw object files into memory while packing if possible

only open raw objects if necessary while writing out pack file data significantly speeds up the "writing pack: " step of gotadmin pack

use time-based rate-limiting for gotadmin progress output Suggested by naddy some time ago. ok tracey

use up to 128 delta chain elements again; creates smaller packs at same speed

try only 3 delta base candidates instead of 10 to speed up packing Tests by kn, thomas_adam and myself made on various repositories indicate that 3 is a good choice. Tyring 10 deltas is much slower and does not result in significantly smaller pack files.

move encode_delta() in pack_create.c to eliminate a forward declaration

while packing, store encoded deltas in a temporary file instead of in memory

limit delta chain length in newly created pack files to 32 deltas Our former limit was 128 which is fairly high. Git uses 50 by default. A smaller limit results in slightly larger pack files but makes both packing and unpacking faster.

raw object blocksize and read buffer were unused; remove them

cache raw objects in order to speed up gotadmin pack