MergedReftable is not used as an AutoCloseable, because closing tables
is currently handled by DfsReftableStack#close.
Encode that a MergedReftable is a list of ReftableReaders. The previous
code suggested that we could form nested trees of MergedReftables,
which is not how we use reftables.
Change-Id: Icbe2fee8a5a12373f45fc5f97d8b1a2b14231c96
Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>