Suggest using `git range-diff` (#1092)

I have found it to be very helpful when rebasing.

1 files changed, 60 insertions, 0 deletions

diff --git a/src/doc/rustc-dev-guide/src/git.md b/src/doc/rustc-dev-guide/src/git.md
index e533426cfca..e1b0e02926d 100644
--- a/src/doc/rustc-dev-guide/src/git.md
+++ b/src/doc/rustc-dev-guide/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 +264,65 @@ 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 rebase, and before pushing up your changes, you may want to
+review the changes between your old branch and your new one. You can do that
+with `git range-diff master @{upstream} HEAD`.
+
+The first argument to `range-diff`, `master` in this case, is the base revision
+that you're comparing your old and new branch against. The second argument is
+the old version of your branch; in this case, `@upstream` means the version that
+you've pushed to GitHub, which is the same as what people will see in your pull
+request. Finally, the third argument to `range-diff` is the *new* version of
+your branch; in this case, it is `HEAD`, which is the commit that is currently
+checked-out in your local repo.
+
+Note that you can also use the equivalent, abbreviated form `git range-diff
+master @{u} HEAD`.
+
+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.
+
+Here's an example of `git range-diff` output (taken from [Git's
+docs][range-diff-example-docs]):
+
+```
+-:  ------- > 1:  0ddba11 Prepare for the inevitable!
+1:  c0debee = 2:  cab005e Add a helpful message at the start
+2:  f00dbal ! 3:  decafe1 Describe a bug
+    @@ -1,3 +1,3 @@
+     Author: A U Thor <author@example.com>
+
+    -TODO: Describe a bug
+    +Describe a bug
+    @@ -324,5 +324,6
+      This is expected.
+
+    -+What is unexpected is that it will also crash.
+    ++Unexpectedly, it also crashes. This is a bug, and the jury is
+    ++still out there how to fix it best. See ticket #314 for details.
+
+      Contact
+3:  bedead < -:  ------- TO-UNDO
+```
+
+(Note that `git range-diff` output in your terminal will probably be easier to
+read than in this example because it will have colors.)
+
+Another feature of `git range-diff` is that, unlike `git diff`, it will also
+diff commit messages. This feature can be useful when amending several commit
+messages so you can make sure you changed the right parts.
+
+`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