Suggest using git range-diff

I have found it to be very helpful when rebasing.

src/git.md

@@ -217,6 +217,7 @@ there are no glaring errors.
 Once you're all done fixing the conflicts, you need to stage the files that had
 conflicts in them via `git add`. Afterwards, run `git rebase --continue` to let
 Git know that you've resolved the conflicts and it should finish the rebase.
+
 Once the rebase has succeeded, you'll want to update the associated branch on
 your fork with `git push --force-with-lease`.
 
@@ -263,6 +264,26 @@ You also may want to squash just the last few commits together, possibly
 because they only represent "fixups" and not real changes. For example,
 `git rebase --interactive HEAD~2` will allow you to edit the two commits only.
 
+### `git range-diff`
+
+After completing a complicated rebase, or even a relatively simple one, you may
+want to review the changes between your old branch and your new one. You can do
+that with the `git range-diff master @{u} HEAD` command, which can be very
+helpful when you had a complicated rebase and you want to make sure you changed
+the right things.
+
+Unlike in regular Git diffs, you'll see a `-` or `+` next to another `-` or `+`
+in the range-diff output. The marker on the left indicates a change between the
+old branch and the new branch, and the marker on the right indicates a change
+you've committed. So, you can think of a range-diff as a "diff of diffs" since
+it shows you the differences between your old diff and your new diff.
+
+`git range-diff` is a very useful command, but note that it can take some time
+to get used to its output format. You may also find Git's documentation on the
+command useful, especially [their "Examples" section][range-diff-example-docs].
+
+[range-diff-example-docs]: https://git-scm.com/docs/git-range-diff#_examples
+
 ## No-Merge Policy
 
 The rust-lang/rust repo uses what is known as a "rebase workflow." This means