diff and merge configuration panel help allows selection out of variables
Right now the preferences for diff and merge for external tools has a help modal dialog that is difficult to work with. Building the command line is repetition of modify and click on the help to make sure things are being typed correctly.
Suggest that instead it's a drop down dialog that populates the variable into the edit box as selected.
Alex Lian commented
One extra suggestion: Offer a "test" 2-way diff and 3-way merge with canned files to test the command line options.