Merge branch 'jn/document-rebase-i-p-limitation' into maint

* jn/document-rebase-i-p-limitation:
  rebase -i -p: document shortcomings

Documentation/git-rebase.txt

@@ -310,6 +310,11 @@ link:howto/revert-a-faulty-merge.txt[revert-a-faulty-merge How-To] for details).
 -p::
 --preserve-merges::
 	Instead of ignoring merges, try to recreate them.
++
+This uses the `--interactive` machinery internally, but combining it
+with the `--interactive` option explicitly is generally not a good
+idea unless you know what you are doing (see BUGS below).
+
 
 --root::
 	Rebase all commits reachable from <branch>, instead of
@@ -611,6 +616,28 @@ The ripple effect of a "hard case" recovery is especially bad:
 case" recovery too!
 
 
+BUGS
+----
+The todo list presented by `--preserve-merges --interactive` does not
+represent the topology of the revision graph.  Editing commits and
+rewording their commit messages should work fine, but attempts to
+reorder commits tend to produce counterintuitive results.
+
+For example, an attempt to rearrange
+------------
+1 --- 2 --- 3 --- 4 --- 5
+------------
+to
+------------
+1 --- 2 --- 4 --- 3 --- 5
+------------
+by moving the "pick 4" line will result in the following history:
+------------
+	3
+       /
+1 --- 2 --- 4 --- 5
+------------
+
 Authors
 ------
 Written by Junio C Hamano <gitster@pobox.com> and

t/t3404-rebase-interactive.sh

@@ -181,6 +181,12 @@ test_expect_success '-p handles "no changes" gracefully' '
 	test $HEAD = $(git rev-parse HEAD)
 '
 
+test_expect_failure 'exchange two commits with -p' '
+	FAKE_LINES="2 1" git rebase -i -p HEAD~2 &&
+	test H = $(git cat-file commit HEAD^ | sed -ne \$p) &&
+	test G = $(git cat-file commit HEAD | sed -ne \$p)
+'
+
 test_expect_success 'preserve merges with -p' '
 	git checkout -b to-be-preserved master^ &&
 	: > unrelated-file &&