The "test-backlinks" command:

Usage: fossil test-backlinks SRCTYPE SRCID ?OPTIONS? INPUT-FILE

Read the content of INPUT-FILE and pass it into the backlink_extract()
routine.  But instead of adding backlinks to the backlink table,
just print them on stdout.  SRCID and SRCTYPE are integers.

Options:
   --mtime DATETIME        Use an alternative date/time.  Defaults to the
                           current date/time.
   --mimetype TYPE         Use an alternative mimetype.