config: clarify compression defaults
config: clarify compression defaults

* Clarify that core.compression provides a system-wide default to
  other compression parameters.

* Explain that the default for pack.compression, -1, is "a default
  compromise between speed and compression (currently equivalent
  to level 6)" according to zlib.h.

Signed-off-by: Brian Downing <47d0c8984e39bbcb35e626c3cdbd84e5cd88ece4@lavos.net>
Acked-by: Nicolas Pitre <d659c10e27d52b00987b65e85d99bce5480adcae@cam.org>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git