config: clarify memory ownership in git_config_string()

The out parameter of `git_config_string()` is a `const char **` even
though we transfer ownership of memory to the caller. This is quite
misleading and has led to many memory leaks all over the place. Adapt
the parameter to instead be `char **`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rebase.c

@@ -83,7 +83,7 @@ static const char *action_names[] = {
 struct rebase_options {
 	enum rebase_type type;
 	enum empty_type empty;
-	const char *default_backend;
+	char *default_backend;
 	const char *state_dir;
 	struct commit *upstream;
 	const char *upstream_name;