config: avoid a funny sentinel value "a^"

Introduce CONFIG_REGEX_NONE as a more explicit sentinel value to say
"we do not want to replace any existing entry" and use it in the
implementation of "git config --add".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -1233,6 +1233,8 @@ extern int update_server_info(int);
 #define CONFIG_INVALID_PATTERN 6
 #define CONFIG_GENERIC_ERROR 7
 
+#define CONFIG_REGEX_NONE ((void *)1)
+
 struct git_config_source {
 	unsigned int use_stdin:1;
 	const char *file;