Fix progress reporting of CLUSTER / VACUUM FULL

Fix progress reporting of CLUSTER / VACUUM FULL

The progress state was being clobbered once the first index completed
being rebuilt, causing the final phases of the operation not show
anything in the progress view.  This was inadvertently broken in
03f9e5cba0ee, which added progress tracking for REINDEX.

(The reason this bugfix is this small is that I had already noticed this
problem when writing monitoring for CREATE INDEX, and had already worked
around it, as can be seen in discussion starting at
https://postgr.es/m/20190329150218.GA25010@alvherre.pgsql Fixing the
problem is just a matter of fixing one place touched by the REINDEX
monitoring.)

Reported by: Álvaro Herrera
Author: Álvaro Herrera
Discussion: https://postgr.es/m/20190801184333.GA21369@alvherre.pgsql