I often need to diff two files that are already opened in Sublime so I created a simple Merge plugin (available in the repo)
For simplicity, this command will diff/merge the last two selected files.
To use: click on view A, click on view B, press Ctrl+Alt+D. WinMerge will be launched with files A and B.
You need to have WinMerge installed: http://winmerge.org/