strbuf: factor out strbuf_expand_step()

Extract the part of strbuf_expand that finds the next placeholder into a
new function.  It allows to build parsers without callback functions and
the overhead imposed by them.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/branch.c

@@ -366,17 +366,8 @@ static const char *quote_literal_for_format(const char *s)
 	static struct strbuf buf = STRBUF_INIT;
 
 	strbuf_reset(&buf);
-	while (*s) {
-		const char *ep = strchrnul(s, '%');
-		if (s < ep)
-			strbuf_add(&buf, s, ep - s);
-		if (*ep == '%') {
-			strbuf_addstr(&buf, "%%");
-			s = ep + 1;
-		} else {
-			s = ep;
-		}
-	}
+	while (strbuf_expand_step(&buf, &s))
+		strbuf_addstr(&buf, "%%");
 	return buf.buf;
 }