diff: accept -- when using --no-index

diff: accept -- when using --no-index

Accept -- as an "end of options" marker even when using --no-index.
Previously, the -- triggered a "normal" index/tree diff and subsequently
failed because of the unrecognized (in that mode) --no-index.

Note that the second loop can treat '--' as a normal option, because
the preceding checks ensure it is the third-to-last argument.

While at it, fix the parsing of "-q" option in --no-index mode as well.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>