index: reduce size of compressed indexes

use the same index size for compressed and uncompressed indexes.
Otherwise, decoding the index of a compressed repository requires
significantly more memory.

cmd/restic/cmd_repair_index_integration_test.go

@@ -68,7 +68,7 @@ func TestRebuildIndexAlwaysFull(t *testing.T) {
 	defer func() {
 		index.IndexFull = indexFull
 	}()
-	index.IndexFull = func(*index.Index, bool) bool { return true }
+	index.IndexFull = func(*index.Index) bool { return true }
 	testRebuildIndex(t, nil)
 }