quote: simplify internals of dequoting

quote: simplify internals of dequoting

Our sq_dequote_to_argv_internal() helper was wrapped by the to_argv()
and to_strvec() forms. Now that we have only the latter, we can stop
wrapping it and drop the argv-only bits.

Note that in theory sq_dequote_to_strvec() could take a const input
string, which would be friendlier to its callers. We couldn't do that
with the to_argv() form because it reused the input string to hold the
output elements. But since we're built on sq_dequote_step(), which
munges the input, we'd have to rework the parser. Since no callers care
about it currently, we'll leave that for another day.

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