diff: don't run pager if user asked for a diff style exit code

diff: don't run pager if user asked for a diff style exit code

As Wincent Colaiuta found out, it's a bit unexpected for git diff to
start a pager even when the --quiet option is specified.  The problem
is that the pager hides the return code -- which is the only output
we're interested in in this case.

Push pager setup down into builtin-diff.c and don't start the pager
if --exit-code or --quiet (which implies --exit-code) was specified.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>