Repository: list flat folders without a type check per entry

Opening a repository lists index/, which holds one file per pack.
StorageBackend::list() checked every entry with is_dir(), and on a
remote storage each check is a round trip. Over SFTP to a Hetzner Storage
Box (33 ms) with a 60 GiB repository, occ backup:run (30 s steps) spent
nearly every step on these requests and uploaded 45 KiB/s; with this
change it uploads 5.8 MB/s. The cost grows with the number of packs.

index/, snapshots/, catalog/ and trash/info/ only ever hold objects:
list them with the new IBackend::listFiles(), one listing without
per-entry checks.

lib/Repository/Catalog.php

@@ -128,7 +128,7 @@ final class Catalog {
 
 	/** @return list<int> */
 	private function generations(): array {
-		return array_map(fn ($p) => (int)basename($p), $this->backend->list('catalog'));
+		return array_map(fn ($p) => (int)basename($p), $this->backend->listFiles('catalog'));
 	}
 
 	private function path(int $gen): string {