do not run pager with diff --no-index --quiet

do not run pager with diff --no-index --quiet

There is no point in running a pager when --quiet is given,
since we are producing no output. The regular diff code path
handles this already, because --quiet implies --exit-code,
and we check for --exit-code when deciding not to run the
pager.

However, the "quiet implies exit-code" logic is done in
diff_setup_done, and the no-index code path sets up its
pager before running diff_setup_done, and misses this case.

We can fix this by reordering our initialization.
Currently we do:

  1. read command line arguments into diff_options

  2. Set pager if EXIT_CODE not requested

  3. always set EXIT_CODE, since we are emulating
     traditional diff

  4. call diff_setup_done

We can fix the problem by moving pager initialization (step
2) after step 4. But step 3 must come after step 2 (since we
want to know whether the _user_ requested --exit-code, not
whether we turned it on unconditionally). So we must move
both.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>