There is an indentation error in the Dissolve.py script that writes out the intermediate object on every merge with an input feature rather than just once at the end of the processing. This results in very large output file sizes.

I have also edited the defineCharacteristics function to allow the tool input to accept polyline features as well as polygons, in line with the fTools UI.
Merge pull request #1236 from aharfoot/master

fix identation in Processing Dissolve tool (fix #9764)
add polylines support