Allow changing the diff tool used by `flake check`

**Is your feature request related to a problem? Please describe.** At the moment, treefmt-nix overwrites the default git config when running `nix flake check`. This is understandable given the possibility of sensitive options or those that require user-interaction, however it also resets the diff tool used. **Describe the solution you'd like** An option to set the diff tool to a custom program, such as [difftastic](https://github.com/Wilfred/difftastic) (pkgs.difftastic), would be useful. **Describe alternatives you've considered** I was able to patch this in manually, though obviously this isn't an ideal solution. ```patch diff --git a/module-options.nix b/module-options.nix index ad83f35..a19d9c7 100644 --- a/module-options.nix +++ b/module-options.nix @@ -315,6 +315,7 @@ in pkgs.git pkgs.git-lfs config.build.wrapper + pkgs.difftastic ]; meta.description = "Check that the project tree is formatted"; } @@ -334,6 +335,13 @@ in email = nix@localhost [init] defaultBranch = main + [core] + conflictStyle = "diff3" + [diff] + external = "${pkgs.difftastic}/bin/difft " + tool = "difftastic" + [difftool "difftastic"] + cmd = "${pkgs.difftastic}/bin/difft $LOCAL $REMOTE" EOF git init --quiet git add . ``` **Additional context** <!-- Add any other context or screenshots about the feature request here. -->