Correct handling of project re-resolving with a new build root.
Correct handling of project re-resolving with a new build root.

In that case, we must ignore the existing project.
In particular, we must acquire a new build graph lock, as the
existing one protects a different build graph file.

Change-Id: I6f84d207959c94d082d9966c85d7b86a14ecfeb3
Reviewed-by: Joerg Bornemann <47777b9406dbddd02a3fdca8341789a013805a11@digia.com>

qtproject/qt-labs-qbs,villytiger/qbs,gatzka/qt-labs-qbs,villytiger/qbs,qt-labs/qbs,villytiger/qbs,gatzka/qt-labs-qbs,Distrotech/qbs,qtproject/qt-labs-qbs,gatzka/qt-labs-qbs,qtproject/qt-labs-qbs,gatzka/qt-labs-qbs,qt-labs/qbs,Distrotech/qbs,qtproject/qt-labs-qbs,villytiger/qbs,qtproject/qt-labs-qbs,gatzka/qt-labs-qbs,qt-labs/qbs,qt-labs/qbs,Distrotech/qbs,gatzka/qt-labs-qbs,qtproject/qt-labs-qbs,gatzka/qt-labs-qbs,qt-labs/qbs,Distrotech/qbs,qt-labs/qbs,Distrotech/qbs,villytiger/qbs,villytiger/qbs,qt-labs/qbs,Distrotech/qbs,qtproject/qt-labs-qbs,Distrotech/qbs