Yes with this patch the error is no longer present, but the deleted file
is not displayed in the*log-edit-files* buffer, unlike if you run
vc-next-action from*vc-dir*. Although this is another bug report.
Right, that's still a remaining problem. diff-find-file-name doesn't really
want to "find" files that don't exist on disk, so it might need
a replacement for such cases.
Indeed, the problem is that diff-find-file-name doesn't get the name
of the deleted file even when called with OLD=t:
;; Use file-regular-p to avoid
;; /dev/null, directories, etc.
((or (null file) (file-regular-p file))
file)