VcsManager: Clear topLevel directory when necessary
VcsManager: Clear topLevel directory when necessary

Clear the topLevelDirectory when being asked to find the version
control system responsible for directory "".

Change-Id: I8806ebff1200f0fc936715ffab94acf1f10cb386
Reviewed-by: Tobias Hunger <012f8d714267bccc9d4766fbeace9f175fcc70c4@digia.com>

maui-packages/qt-creator,maui-packages/qt-creator,kuba1/qtcreator,AltarBeastiful/qt-creator,maui-packages/qt-creator,martyone/sailfish-qtcreator,omniacreator/qtcreator,xianian/qt-creator,AltarBeastiful/qt-creator,martyone/sailfish-qtcreator,colede/qtcreator,kuba1/qtcreator,martyone/sailfish-qtcreator,colede/qtcreator,darksylinc/qt-creator,martyone/sailfish-qtcreator,amyvmiwei/qt-creator,omniacreator/qtcreator,martyone/sailfish-qtcreator,kuba1/qtcreator,darksylinc/qt-creator,kuba1/qtcreator,danimo/qt-creator,farseerri/git_code,kuba1/qtcreator,AltarBeastiful/qt-creator,farseerri/git_code,AltarBeastiful/qt-creator,maui-packages/qt-creator,darksylinc/qt-creator,danimo/qt-creator,omniacreator/qtcreator,xianian/qt-creator,xianian/qt-creator,danimo/qt-creator,danimo/qt-creator,colede/qtcreator,farseerri/git_code,farseerri/git_code,colede/qtcreator,Distrotech/qtcreator,xianian/qt-creator,farseerri/git_code,Distrotech/qtcreator,danimo/qt-creator,martyone/sailfish-qtcreator,xianian/qt-creator,kuba1/qtcreator,colede/qtcreator,danimo/qt-creator,danimo/qt-creator,martyone/sailfish-qtcreator,omniacreator/qtcreator,xianian/qt-creator,amyvmiwei/qt-creator,AltarBeastiful/qt-creator,kuba1/qtcreator,omniacreator/qtcreator,AltarBeastiful/qt-creator,darksylinc/qt-creator,amyvmiwei/qt-creator,kuba1/qtcreator,farseerri/git_code,maui-packages/qt-creator,darksylinc/qt-creator,amyvmiwei/qt-creator,amyvmiwei/qt-creator,farseerri/git_code,colede/qtcreator,colede/qtcreator,AltarBeastiful/qt-creator,xianian/qt-creator,Distrotech/qtcreator,danimo/qt-creator,danimo/qt-creator,omniacreator/qtcreator,Distrotech/qtcreator,Distrotech/qtcreator,AltarBeastiful/qt-creator,kuba1/qtcreator,amyvmiwei/qt-creator,maui-packages/qt-creator,darksylinc/qt-creator,xianian/qt-creator,omniacreator/qtcreator,farseerri/git_code,amyvmiwei/qt-creator,maui-packages/qt-creator,amyvmiwei/qt-creator,martyone/sailfish-qtcreator,Distrotech/qtcreator,xianian/qt-creator,martyone/sailfish-qtcreator,Distrotech/qtcreator,darksylinc/qt-creator,darksylinc/qt-creator