diff --check: minor fixups

diff --check: minor fixups

There is no reason --exit-code and --check-diff must be mutually
exclusive, so assign different bits to different results and allow them
to be returned from the command.  Introduce diff_result_code() to factor
out the common code to decide final status code based on diffopt
settings and use it everywhere.

Update tests to match the above fix.

Turning pager off when "diff --check" is used is a regression.

Signed-off-by: Junio C Hamano <gitster@pobox.com>