Merge branch 'nd/gc-aggressive'

Allow tweaking the maximum length of the delta-chain produced by
"gc --aggressive".

* nd/gc-aggressive:
environment.c: fix constness for odb_pack_keep()
gc --aggressive: make --depth configurable

diff --cc Documentation/config.txt
Simple merge

diff --cc builtin/gc.c
Simple merge

diff --cc git-compat-util.h
Simple merge