Improve register{Added,Removed}FileTag.
Improve register{Added,Removed}FileTag.

- Do not do double hash look-up.
- Do not leave empty lists in the hash.

Change-Id: I09c6c0917fc4b97032a4a48ba62419d9ebd236e5
Reviewed-by: Joerg Bornemann <47777b9406dbddd02a3fdca8341789a013805a11@digia.com>

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