Add Use External Diff Tool Like Difftastic as a Git TIL

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1546 TILs and counting..._
+_1547 TILs and counting..._
 
 ---
 
@@ -383,6 +383,7 @@ _1546 TILs and counting..._
 - [Untrack A Directory Of Files Without Deleting](git/untrack-a-directory-of-files-without-deleting.md)
 - [Untrack A File Without Deleting It](git/untrack-a-file-without-deleting-it.md)
 - [Update The URL Of A Remote](git/update-the-url-of-a-remote.md)
+- [Use External Diff Tool Like Difftastic](git/use-external-diff-tool-like-difftastic.md)
 - [Using Commands With A Relative Date Format](git/using-commands-with-a-relative-date-format.md)
 - [Verbose Commit Message](git/verbose-commit-message.md)
 - [Viewing A File On Another Branch](git/viewing-a-file-on-another-branch.md)

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

@@ -0,0 +1,23 @@
+# 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`.