rebase-i-p: if todo was reordered use HEAD as the rewritten parent

This seems like the best guess we can make until git sequencer marks are
available. That being said, within the context of re-ordering a commit before
its parent in todo, I think applying it on top of the current commit seems like
a reasonable assumption of what the user intended.

Signed-off-by: Stephen Haberman <stephen@exigencecorp.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--interactive.sh

@@ -194,6 +194,15 @@ pick_one_preserving_merges () {
 		if test -f "$REWRITTEN"/$p
 		then
 			new_p=$(cat "$REWRITTEN"/$p)
+
+			# If the todo reordered commits, and our parent is marked for
+			# rewriting, but hasn't been gotten to yet, assume the user meant to
+			# drop it on top of the current HEAD
+			if test -z "$new_p"
+			then
+				new_p=$(git rev-parse HEAD)
+			fi
+
 			test $p != $new_p && fast_forward=f
 			case "$new_parents" in
 			*$new_p*)

t/t3411-rebase-preserve-around-merges.sh

@@ -80,7 +80,7 @@ test_expect_success 'setup' '
 #       \        /
 #        -- C1 --
 #
-test_expect_failure 'squash F1 into D1' '
+test_expect_success 'squash F1 into D1' '
 	FAKE_LINES="1 squash 3 2" git rebase -i -p B1 &&
 	test "$(git rev-parse HEAD^2)" = "$(git rev-parse branch)" &&
 	test "$(git rev-parse HEAD~2)" = "$(git rev-parse B1)" &&
@@ -99,7 +99,7 @@ test_expect_failure 'squash F1 into D1' '
 #
 # And rebase G1..M1 onto E2
 
-test_expect_failure 'rebase two levels of merge' '
+test_expect_success 'rebase two levels of merge' '
 	git checkout -b branch2 A1 &&
 	touch g &&
 	git add g &&