git: use difftastic by default

As described in
https://difftastic.wilfred.me.uk/git.html#difftastic-by-default, the
`--ext-diff` option needs to be used for difftastic to be used on
commands other than `git diff`, such as `git show` or `git log`.

> This only applies to git diff. For other git commands, you still need
to specify --ext-diff, or use an alias as described above.

```shell
$ git diff
$ git show HEAD --ext-diff
$ git log -p --ext-diff
```

> Conversely, if you need to copy/paste a diff in the standard diff
format, you can use git diff --no-ext-diff.

lib/shared/modules/zsh.nix

@@ -20,6 +20,11 @@
           else
             (cd "''${ROOT}" && eval "''${@}")
           fi
+        elif [[ "''$1" == "log" || "''$1" == "show"  ]]; then
+          local command="$1"
+          shift 1
+
+          ${pkgs.git}/bin/git "$command" --ext-diff "$@"
         else
           ${pkgs.git}/bin/git "''${@}"
         fi