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.c

@@ -116,7 +116,7 @@ void sq_append_quote_argv_pretty(struct strbuf *dst, const char **argv)
 	}
 }
 
-static char *sq_dequote_step(char *arg, char **next)
+char *sq_dequote_step(char *arg, char **next)
 {
 	char *dst = arg;
 	char *src = arg;
@@ -153,11 +153,8 @@ static char *sq_dequote_step(char *arg, char **next)
 			}
 		/* Fallthrough */
 		default:
-			if (!next || !isspace(*src))
+			if (!next)
 				return NULL;
-			do {
-				c = *++src;
-			} while (isspace(c));
 			*dst = 0;
 			*next = src;
 			return arg;
@@ -182,6 +179,14 @@ static int sq_dequote_to_argv_internal(char *arg,
 		char *dequoted = sq_dequote_step(next, &next);
 		if (!dequoted)
 			return -1;
+		if (next) {
+			char c;
+			if (!isspace(*next))
+				return -1;
+			do {
+				c = *++next;
+			} while (isspace(c));
+		}
 		if (argv) {
 			ALLOC_GROW(*argv, *nr + 1, *alloc);
 			(*argv)[(*nr)++] = dequoted;

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,