Storage limit per location

S3 has no bucket quota, so NG Backup enforces one: before a pack is
uploaded, the stored size plus the pack must stay under the location's
limit, otherwise the backup stops with an error (and administrators are
notified through the failed run). Repacking during prune is exempt, as it
is how space is freed. occ backup:target:limit NAME [SIZE|none].

Also ignore credential files in the repository.

tests/Unit/Repository/RepositoryTest.php

@@ -146,4 +146,16 @@ class RepositoryTest extends TestCase {
 		$this->assertFileDoesNotExist(dirname($target, 2) . '/escaped.txt');
 		$this->assertFileDoesNotExist(dirname($target) . '/escaped.txt');
 	}
+
+	public function testStorageLimitStopsTheBackup(): void {
+		$repo = Repository::init($this->backend, 'pw');
+		$repo->setMaxBytes(6 * 1048576); // less than the test tree
+		try {
+			$repo->backupDirectory($this->src);
+			$this->fail('backup should hit the limit');
+		} catch (\OCA\NgBackup\Repository\QuotaExceededException) {
+		}
+		$this->assertLessThanOrEqual(6 * 1048576, $repo->storedBytes());
+		$this->assertSame([], $repo->listSnapshots(), 'no snapshot from an incomplete backup');
+	}
 }