quote: implement "sq_dequote_many" to unwrap many args in one string

quote: implement "sq_dequote_many" to unwrap many args in one string

The sq_dequote() function does not allow parsing a string with more than
one single-quoted parameter easily; use its code to implement a new API
sq_dequote_step() to allow the caller iterate through such a string to
parse them one-by-one.  The original sq_dequote() becomes a thin wrapper
around it.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>