Start with an empty page

[azzamsa]

Meld is allowed to start with an empty page.
Sometimes I use it to show the diff between non-technical documents from my clipboard.
But, diffedit3 refuses to open without initial documents.

🦄 diffedit3
Must have 2 or 3 dirs to compare, got 0 dirs instead

[ilyagr]

This should be possible. The main limitation is that there is no "save as" yet. I'm not sure whether it's better to show an error when the user tries to save or to create three files in a temporary dir, maybe.

This is also related to another feature I have in mind, to allow passing file paths to diffedit3 instead of dirs, so that you get one set of three panes that gets maximized by default.

[ilyagr]

This case would also be special in that it'd be the only case where the left/right panes are editable. That's probably OK, but requires some minor change to the front-end in addition to the back-end changes.