notes: don't leak memory in git_config_get_notes_strategy

This function asks for the value of a configuration and after
using the value does not have to retain ownership of it.
git_config_get_string_const() however is a function to get a
copy of the value, but we forget to free it before we return.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/notes.c

@@ -741,13 +741,14 @@ static int merge_commit(struct notes_merge_options *o)
 static int git_config_get_notes_strategy(const char *key,
 					 enum notes_merge_strategy *strategy)
 {
-	const char *value;
+	char *value;
 
-	if (git_config_get_string_const(key, &value))
+	if (git_config_get_string(key, &value))
 		return 1;
 	if (parse_notes_merge_strategy(value, strategy))
 		git_die_config(key, "unknown notes merge strategy %s", value);
 
+	free(value);
 	return 0;
 }