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>

quote.h

@@ -42,12 +42,26 @@ void sq_quote_buf_pretty(struct strbuf *, const char *src);
 void sq_quote_argv_pretty(struct strbuf *, const char **argv);
 void sq_append_quote_argv_pretty(struct strbuf *dst, const char **argv);
 
-/* This unwraps what sq_quote() produces in place, but returns
+/*
+ * This unwraps what sq_quote() produces in place, but returns
  * NULL if the input does not look like what sq_quote would have
- * produced.
+ * produced (the full string must be a single quoted item).
  */
 char *sq_dequote(char *);
 
+/*
+ * Like sq_dequote(), but dequote a single item, and leave "next" pointing to
+ * the next character. E.g., in the string:
+ *
+ *   'one' 'two' 'three'
+ *
+ * after the first call, the return value would be the unquoted string "one",
+ * with "next" pointing to the space between "one" and "two"). The caller is
+ * responsible for advancing the pointer to the start of the next item before
+ * calling sq_dequote_step() again.
+ */
+char *sq_dequote_step(char *src, char **next);
+
 /*
  * Same as the above, but can be used to unwrap many arguments in the
  * same string separated by space. Like sq_quote, it works in place,