Once a file deletion has been committed, it is extremely difficult to undo this. The only way I managed to do it was to find the revision where it occurred in the History Viewer, choose "Revert changes from this revision" (thereby reverting *all* the changes done there) and then again reverting the other changes.

Much simpler if I could select the "delete file" action in the History Viewer's changed paths and choose "Revert".