[REG: 4.8.2 -> 4.9]: Perforce integration doesn't work in some cases anymore

Project structure like this:

/project/
/project/src

Qt Creator project files are in /project/, but they are not part of a Perforce repository. /project/src contains sources and it is a Perforce repository.

With 4.8.2 this reportedly worked fine if Qt Creator was started under the src directory.

With 4.9, Qt Creator does something different. It invokes the "p4 client -o" call first in /project/src and then under /project/ when the project has been loaded. This latter call succeeds, but returns invalid client value which might be causing this issue.