quote: make sq_dequote_step() a public function

quote: make sq_dequote_step() a public function

We provide a function for dequoting an entire string, as well as one for
handling a space-separated list of quoted strings. But there's no way
for a caller to parse a string like 'foo'='bar', even though it is easy
to generate one using sq_quote_buf() or similar.

Let's make the single-step function available to callers outside of
quote.c. Note that we do need to adjust its implementation slightly: it
insists on seeing whitespace between items, and we'd like to be more
flexible than that. Since it only has a single caller, we can move that
check (and slurping up any extra whitespace) into that caller.

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