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/Backend/IBackend.php

@@ -37,5 +37,15 @@ interface IBackend {
 	/** @return list<string> object paths below $prefix (recursive) */
 	public function list(string $prefix): array;
 
+	/**
+	 * Object paths directly in $dir, for a folder that only holds objects (index/, snapshots/,
+	 * catalog/, trash/info/). One listing, without checking the type of every entry: on a remote
+	 * storage list() costs a round trip per entry, which for index/ (one file per pack) grows
+	 * with the size of the repository.
+	 *
+	 * @return list<string>
+	 */
+	public function listFiles(string $dir): array;
+
 	public function delete(string $path): void;
 }

lib/Backend/LocalBackend.php

@@ -107,6 +107,22 @@ final class LocalBackend implements IBackend {
 		return $result;
 	}
 
+	public function listFiles(string $dir): array {
+		$base = $this->abs($dir);
+		if (!is_dir($base)) {
+			return [];
+		}
+		$result = [];
+		$names = scandir($base);
+		foreach ($names === false ? [] : $names as $name) {
+			if ($name !== '.' && $name !== '..' && !StorageBackend::isTemporary($name)) {
+				$result[] = trim($dir, '/') . '/' . $name;
+			}
+		}
+		sort($result);
+		return $result;
+	}
+
 	public function delete(string $path): void {
 		@unlink($this->abs($path));
 	}

lib/Backend/StorageBackend.php

@@ -133,6 +133,27 @@ final class StorageBackend implements IBackend {
 		}
 	}
 
+	public function listFiles(string $dir): array {
+		$abs = $this->abs($dir);
+		if (!$this->storage->is_dir($abs)) {
+			return []; // not created yet
+		}
+		$dh = $this->storage->opendir($abs);
+		if ($dh === false) {
+			return [];
+		}
+		$cut = $this->base === '' ? 0 : strlen($this->base) + 1;
+		$result = [];
+		while (($name = readdir($dh)) !== false) {
+			if ($name !== '.' && $name !== '..' && !self::isTemporary($name)) {
+				$result[] = substr($abs . '/' . $name, $cut);
+			}
+		}
+		closedir($dh);
+		sort($result);
+		return $result;
+	}
+
 	private function listInto(string $dir, array &$result): void {
 		if (!$this->storage->is_dir($dir)) {
 			return;