[BUGFIX] Suppress double page entry in temporary mounted pagetree

This patch suppresses that some pages will be shown as subpages of
themselves while defining a temporary mountpoint in the pagetree and then
searching in the mounted tree.

Fixes: #47123
Releases: 6.1, 6.0, 4.7
Change-Id: I2d6f895b6b6fd110e9a4fbc163295e8c992fdb61
Reviewed-on: https://review.typo3.org/19977
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind