pgbench: Ensure previous progress message is fully cleared when updating.

pgbench: Ensure previous progress message is fully cleared when updating.

During pgbench's table initialization, progress updates could display
leftover characters from the previous message if the new message
was shorter. This commit resolves the issue by appending spaces to
the current message to fully overwrite any remaining characters from
the previous line.

Back-patch to all the supported versions.

Author: Yushi Ogiwara, Tatsuo Ishii, Fujii Masao
Reviewed-by: Tatsuo Ishii, Fujii Masao
Discussion: http://postgr.es/m/9a9b8b95b6a709877ae48ad5b0c59bb9@oss.nttdata.com