Fossil
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.