How backups work

What does this mean? Yours was completely ignored? Mine broke my backup, because it’s wrong.
The original post had it in use. I especially worried about the “source mapping” issue you mention.
I’m afraid the language there is vague enough that I don’t follow it, so I mention the suspect option.

Generally (especially when starting out), sticking close to the defaults is likely to invite less trouble.
The –changed-files option is missing some key information on its use, which you can find in forum:

It would be nice if someone could confirm this, enhance the help text, then get the manual updated.
Possibly a GitHub issue will be needed to get it in the queue for a developer. Sometimes that’s so…