qgis/Quantum-GIS — pull request #323 by sanak (2012-11-09 12:27:54+00:00)
Title: Add line layer support for dissolve function

QgsGeometry.combine method support line merge for line type geometry. (from qgis/Quantum-GIS@092a1bf)

Geoprocessing-dissolve function use QgsGeometry.combine method,
so, I think that dissolve function can support line layers by this modification.