Fix rebase -p --onto

In a rebase with --onto, the correct test for whether we can skip
rewriting a commit is if it is already on top of $ONTO, not $UPSTREAM.
Without --onto, this distinction does not exist and the behavior does
not change.

In a situation with two merged branches on a common base X:

 X---o---o---o---M
  \             /
   x---x---x---x

 Y

if we try to move the branches from their base on X to be based on Y,
so as to get

 X

 Y---o'--o'--o'--M'
  \             /
   x'--x'--x'--x'

then we fail.  The command `git rebase -p --onto Y X M` moves only the
first-parent chain, like so:

 X
  \
   x---x---x---x
                \
 Y---o'--o'--o'--M'

because it mistakenly drops the other branch(es) x---x---x---x from
the TODO file.  This tests and fixes this behavior.

Signed-off-by: Greg Price <price@ksplice.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--interactive.sh

@@ -703,7 +703,7 @@ first and then run 'git rebase --continue' again."
 					preserve=t
 					for p in $(git rev-list --parents -1 $sha1 | cut -d' ' -s -f2-)
 					do
-						if test -f "$REWRITTEN"/$p -a \( $p != $UPSTREAM -o $sha1 = $first_after_upstream \)
+						if test -f "$REWRITTEN"/$p -a \( $p != $ONTO -o $sha1 = $first_after_upstream \)
 						then
 							preserve=f
 						fi

t/t3414-rebase-preserve-onto.sh

@@ -0,0 +1,80 @@
+#!/bin/sh
+#
+# Copyright (c) 2009 Greg Price
+#
+
+test_description='git rebase -p should respect --onto
+
+In a rebase with --onto, we should rewrite all the commits that
+aren'"'"'t on top of $ONTO, even if they are on top of $UPSTREAM.
+'
+. ./test-lib.sh
+
+. ../lib-rebase.sh
+
+# Set up branches like this:
+# A1---B1---E1---F1---G1
+#  \    \             /
+#   \    \--C1---D1--/
+#    H1
+
+test_expect_success 'setup' '
+	test_commit A1 &&
+	test_commit B1 &&
+	test_commit C1 &&
+	test_commit D1 &&
+	git reset --hard B1 &&
+	test_commit E1 &&
+	test_commit F1 &&
+	test_merge G1 D1 &&
+	git reset --hard A1 &&
+	test_commit H1
+'
+
+# Now rebase merge G1 from both branches' base B1, both should move:
+# A1---B1---E1---F1---G1
+#  \    \             /
+#   \    \--C1---D1--/
+#    \
+#     H1---E2---F2---G2
+#      \             /
+#       \--C2---D2--/
+
+test_expect_success 'rebase from B1 onto H1' '
+	git checkout G1 &&
+	git rebase -p --onto H1 B1 &&
+	test "$(git rev-parse HEAD^1^1^1)" = "$(git rev-parse H1)" &&
+	test "$(git rev-parse HEAD^2^1^1)" = "$(git rev-parse H1)"
+'
+
+# On the other hand if rebase from E1 which is within one branch,
+# then the other branch stays:
+# A1---B1---E1---F1---G1
+#  \    \             /
+#   \    \--C1---D1--/
+#    \             \
+#     H1-----F3-----G3
+
+test_expect_success 'rebase from E1 onto H1' '
+	git checkout G1 &&
+	git rebase -p --onto H1 E1 &&
+	test "$(git rev-parse HEAD^1^1)" = "$(git rev-parse H1)" &&
+	test "$(git rev-parse HEAD^2)" = "$(git rev-parse D1)"
+'
+
+# And the same if we rebase from a commit in the second-parent branch.
+# A1---B1---E1---F1----G1
+#  \    \          \   /
+#   \    \--C1---D1-\-/
+#    \               \
+#     H1------D3------G4
+
+test_expect_success 'rebase from C1 onto H1' '
+	git checkout G1 &&
+	git rev-list --first-parent --pretty=oneline C1..G1 &&
+	git rebase -p --onto H1 C1 &&
+	test "$(git rev-parse HEAD^2^1)" = "$(git rev-parse H1)" &&
+	test "$(git rev-parse HEAD^1)" = "$(git rev-parse F1)"
+'
+
+test_done