Add Use External Diff Tool Like Difftastic as a Git TIL

git/use-external-diff-tool-like-difftastic.md

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+# Use External Diff Tool Like Difftastic
+
+Assuming we already have a tool like `difft`
+([difftastic](https://difftastic.wilfred.me.uk/introduction.html)) available on
+our machine, we can use it as a diff viewer for the various `git` commands that
+display a diff.
+
+This requires a manual override which involve two pieces — an inline
+configuration of `diff.external` specifying the binary of the external differ
+and the `--ext-diff` flag which tells these commands to use the external diff
+binary.
+
+Here is what `git show` looks like with `difft`:
+
+```bash
+$ git -c diff.external=difft show --ext-diff
+```
+
+Without the `--ext-diff` flag, it will fallback to the default differ despite
+`diff.external` being set.
+
+See `man git-diff` and friends for the `--ext-diff` flag. See `man git-config`
+for `diff.external`.