fix pager.diff with diff --no-index

fix pager.diff with diff --no-index

git-diff does not rely on the git wrapper to setup its
pager; instead, it sets it up on its own after seeing
whether --quiet or --exit-code has been specified.  After
diff_no_index was split off from cmd_diff, commit b3fde6c
(git diff --no-index: default to page like other diff
frontends, 2008-05-26) duplicated the one-liner from
cmd_diff to turn on the pager.

Later, commit 8f0359f (Allow pager of diff command be
enabled/disabled, 2008-07-21) taught the the version in
cmd_diff to respect the pager.diff config, but the version
in diff_no_index was left behind. This meant that

  git -c pager.diff=0 diff a b

would not use a pager, but

  git -c pager.diff=0 diff --no-index a b

would.  Let's fix it by factoring out a common function.

While we're there, let's update the antiquated comment,
which claims that the pager interferes with propagating the
exit code; this has not been the case since ea27a18 (spawn
pager via run_command interface, 2008-07-22).

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