Fix calculation of latency of pgbench backslash commands.

Fix calculation of latency of pgbench backslash commands.

When we loop back to the top of doCustom after processing a backslash
command, we must reset the "now" timestamp, because that's used to
calculate the time spent executing the previous command.

Report and fix by Fabien Coelho. Backpatch to 9.5, where this was broken.