Commit 8ff1c946 extended TRUNCATE command so that it can also truncate
foreign tables. But it forgot to support tab-complete for TRUNCATE on
foreign tables. That is, previously tab-complete for TRUNCATE displayed
only the names of regular tables.

This commit improves tab-complete for TRUNCATE so that it displays also
the names of foreign tables.

Author: Fujii Masao
Reviewed-by: Bharath Rupireddy
Discussion: https://postgr.es/m/551ed8c1-f531-818b-664a-2cecdab99cd8@oss.nttdata.com