Nathan Hoad's How to: Meld for Git diffs almost worked for me. Two minor things got in the way. First you have to have a change in the selected file or git diff does noting. There is no error message, just nothing happens. It took me a minute to figure out. Sure seems obvious, but I tripped over it. Second, you have to set the execute permission on the python file. The error message made this obvious, still worth noting.