Merge branch 'ew/fast-import-unpack-limit'

"git fast-import" learned the same performance trick to avoid
creating too small a packfile as "git fetch" and "git push" have,
using *.unpackLimit configuration.

* ew/fast-import-unpack-limit:
fast-import: invalidate pack_id references after loosening
fast-import: implement unpack limit

diff --cc Documentation/config.txt
Simple merge

diff --cc fast-import.c
Simple merge

diff --cc t/t9300-fast-import.sh
Simple merge