strbuf: give URL-encoding API a char predicate fn

Allow callers to specify exactly what characters need to be URL-encoded
and which do not. This new API will be taken advantage of in a patch
later in this set.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Matthew DeVore <matvore@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

strbuf.h

@@ -666,8 +666,13 @@ void strbuf_branchname(struct strbuf *sb, const char *name,
  */
 int strbuf_check_branch_ref(struct strbuf *sb, const char *name);
 
+typedef int (*char_predicate)(char ch);
+
+int is_rfc3986_unreserved(char ch);
+int is_rfc3986_reserved_or_unreserved(char ch);
+
 void strbuf_addstr_urlencode(struct strbuf *sb, const char *name,
-			     int reserved);
+			     char_predicate allow_unencoded_fn);
 
 __attribute__((format (printf,1,2)))
 int printf_ln(const char *fmt, ...);