Backup: do not write the index cache inside the read-only dump transaction

A database dump that fills a whole pack (32 MiB) uploads that pack while
the dump transaction is still open, and the pack's index cache row was
inserted on the same connection. PostgreSQL refuses that (25006, read-only
transaction) and the run failed; on other databases the row was rolled
back with the dump transaction. The index now holds those cache writes
back until the transaction is over.

lib/Db/DbDumper.php

@@ -72,6 +72,9 @@ final class DbDumper {
 	public function dumpStep(Repository $repo, array $state, float $deadline, array &$stats): array {
 		$state['steps']++;
 		$sm = $this->db->createSchemaManager();
+		// 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();
 		$this->db->setTransactionIsolation(TransactionIsolationLevel::REPEATABLE_READ);
 		$this->db->beginTransaction();
 		try {
@@ -116,6 +119,7 @@ final class DbDumper {
 			}
 		} finally {
 			$this->db->rollBack();
+			$repo->releaseIndexCache();
 		}
 		$repo->flushPacks();
 		return $state;