Merge branch 'jk/reduce-gc-aggressive-depth' into maint

"git gc --aggressive" used to limit the delta-chain length to 250,
which is way too deep for gaining additional space savings and is
detrimental for runtime performance.  The limit has been reduced to
50.

* jk/reduce-gc-aggressive-depth:
  gc: default aggressive depth to 50

diff --cc Documentation/config.txt
Simple merge

diff --cc builtin/gc.c
Simple merge