Fossil supports external diff commands by setting diff-command for the current repository:

fossil settings diff-command difft

To use difftastic for all repositories, use --global:

fossil settings diff-command --global difft

Skip difftastic on Fossil

If you set difftastic as Fossil's diff command, but you need to use Fossil's internal diff once, use -i to skip difftastic once:

fossil diff -i

If you want to remove difftastic from one repository (or globally), use unset:

fossil unset diff-command

unset also supports the --global flag.