Perforce integration only works when starting qtcreator from the project directory

After enabling the Perforce plugin and setting up the options in the configuration dialog, the perforce integration was still not working (editing a file wouldn't make it writable).

I did some debugging and here's what happens on startup:

Perforce::Internal::PerforcePluginPrivate::getTopLevel: Creating PerforceChecker for workingDirectory ""
 Perforce::Internal::PerforcePluginPrivate::setTopLevel: "Perforce repository: /home/kdab/src/qt-creator/build" 

Obviously this isn't a perforce repository, and getTopLevel/setTopLevel is never called again after opening the last session with a cmake project in perforce, so nothing works.

Same test, but starting QtCreator from the toplevel directory of the project sources:

Perforce::Internal::PerforcePluginPrivate::getTopLevel: Creating PerforceChecker for workingDirectory "" 
Perforce::Internal::PerforcePluginPrivate::setTopLevel: "Perforce repository: /home/kdab/customername/workspacename" 
Perforce::Internal::PerforcePluginPrivate::managesDirectoryFstat: ("fstat", "-m1", [...]) 

=> we're good to go, everything works.

This is debug output I added myself in the QtCreator 8.0.2 sources, but the bug also happens in QtCreator 9 (installed from online binaries)