Backup: release the held index cache writes also when the dump transaction fails to start

holdIndexCache() ran before beginTransaction(), outside the try, and the release came after
rollBack(); an exception from either left later cache writes queued for the rest of the run.

lib/Db/DbDumper.php

@@ -75,6 +75,7 @@ final class DbDumper {
 		// A pack that fills up during the dump is uploaded right away; its index cache row must
 		// wait until the read-only transaction is over.
 		$repo->holdIndexCache();
+		try {
 			$this->db->setTransactionIsolation(TransactionIsolationLevel::REPEATABLE_READ);
 			$this->db->beginTransaction();
 			try {
@@ -119,6 +120,8 @@ final class DbDumper {
 				}
 			} finally {
 				$this->db->rollBack();
+			}
+		} finally {
 			$repo->releaseIndexCache();
 		}
 		$repo->flushPacks();