git-rebase: document suppression of duplicate commits

git-rebase uses format-patch's --ignore-if-in-upstream
option, but we never document the user-visible behavior. The
example is placed near the top of the example list rather
than at the bottom because it is:
  a. a simple example
  b. a reasonably common scenario for many projects (mail
     some patches which get accepted upstream, then rebase)

[sp: Corrected direction of 'HEAD..<upstream>' set comparsion]

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

Documentation/git-rebase.txt

@@ -28,7 +28,10 @@ The current branch is reset to <upstream>, or <newbase> if the
 `git reset --hard <upstream>` (or <newbase>).
 
 The commits that were previously saved into the temporary area are
-then reapplied to the current branch, one by one, in order.
+then reapplied to the current branch, one by one, in order. Note that
+any commits in HEAD which introduce the same textual changes as a commit
+in HEAD..<upstream> are omitted (i.e., a patch already accepted upstream
+with a different commit message or timestamp will be skipped).
 
 It is possible that a merge failure will prevent this process from being
 completely automatic.  You will have to resolve any such merge failure
@@ -62,6 +65,26 @@ would be:
 The latter form is just a short-hand of `git checkout topic`
 followed by `git rebase master`.
 
+If the upstream branch already contains a change you have made (e.g.,
+because you mailed a patch which was applied upstream), then that commit
+will be skipped. For example, running `git-rebase master` on the
+following history (in which A' and A introduce the same set of changes,
+but have different committer information):
+
+------------
+          A---B---C topic
+         /
+    D---E---A'---F master
+------------
+
+will result in:
+
+------------
+                   B'---C' topic
+                  /
+    D---E---A'---F master
+------------
+
 Here is how you would transplant a topic branch based on one
 branch to another, to pretend that you forked the topic branch
 from the latter branch, using `rebase --onto`.