We have a diff dialog for libries.

1. Start JabRef
2. Open https://github.com/JabRef/jabref-demonstration-libraries/blob/main/chocolate/Chocolate.bib
3. Open the same file in a text editor
4. Change the text file
5. Save
6. See JabRef notification

Click on "Review Changes"

See dialog

Wish: "File" > "Merge..." (right above "Import") should

1. Open file chooser for *.bib
2. Call the comparison as called above
3. Show the dialog - should be empty if no changes.

Alternative considered: Import dialog

But this dialog assumes the entries are new...