t4020: test exit code with external diffs

Add tests to check the exit code of git diff with its options --quiet
and --exit-code when using an external diff program.  Currently we
cannot tell whether it found significant changes or not.

While at it, document briefly that --quiet turns off execution of
external diff programs because that behavior surprised me for a moment
while writing the tests.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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