Spotted by Ivo while reviewing the rushed code addition: “
 - if get_fresh_dmesg doens't find any new lines in dmesg, the timestamp is empty
 - next time it runs, it outputs the entire dmesg again
”

It seems it would make sense to check whether $dmesg_file is empty, so
that we don't forget about the previously-saved timestamp.

It seems to me a fix could go through unstable→testing (as stretch), and
be backported for 8.1 (as opposed to re-uploading after RC3, before 8.0).

Mraw,
KiBi.