Steps:
- Use TortoiseSVN's context menu to select "Create Patch"
- On another machine do the same but select "Apply Patch" and select the file generated in step 1.
- A blank merge window is opened.
It looks like this:
The patch file is valid and I can use unix patch to apply it successfully (with some line-ending tinkering).
I'm on Windows 10 and TortoiseSVN/TortoiseMerge 1.9.5