[BUGFIX] Fix page tree mountpoint path

If options.pageTree.showPathAboveMounts is enabled in
UserTS, don't add the name of the mountpoint startpage to
this path.

Resolves: #77734
Releases: master, 7.6
Change-Id: I68da82778921ac37e6b827ab8b6ca21993cf6491
Reviewed-on: https://review.typo3.org/49748


Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>