would close and then re-open rel being truncated.  Depending on the
luck of the draw, the re-opened relcache entry might or might not be
at the same physical location as before.  Unfortunately, if it wasn't
then heap_truncate would crash and burn, because it still had a pointer
at the old location.  Fix is to open and then close rel in
RelationTruncateIndexes, so that rel's refcount never goes to zero
until heap_truncate is done.