diff: diff.context configuration gives default to -U

diff: diff.context configuration gives default to -U

Introduce a configuration variable diff.context that tells
Porcelain commands to use a non-default number of context
lines instead of 3 (the default).  With this variable, users
do not have to keep repeating "git log -U8" from the command
line; instead, it becomes sufficient to say "git config
diff.context 8" just once.

Signed-off-by: Jeff Muizelaar <jmuizelaar@mozilla.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>