Allow pager of diff command be enabled/disabled

See for example, status and show commands. Besides,
Documentation/RelNotes-1.6.0.txt mentions that pager.<cmd>
can be used to enable/disable paging behavior per command.

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>