t/t2021-checkout-last: "checkout -" should work after a rebase finishes

  $ git checkout -

does not work as expected after a rebase.  This is because the
reflog records "checkout" made by "rebase" as its implementation
detail the same way as end-user initiated "checkout", and makes it
count as the branch that was previously checked out.

Add four failing tests documenting this bug: two for a normal rebase,
and another two for an interactive rebase.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t2012-checkout-last.sh

@@ -116,4 +116,38 @@ test_expect_success 'master...' '
 	test "z$(git rev-parse --verify HEAD)" = "z$(git rev-parse --verify master^)"
 '
 
+test_expect_failure '"checkout -" works after a rebase A' '
+	git checkout master &&
+	git checkout other &&
+	git rebase master &&
+	git checkout - &&
+	test "z$(git symbolic-ref HEAD)" = "zrefs/heads/master"
+'
+
+test_expect_failure '"checkout -" works after a rebase A B' '
+	git branch moodle master~1 &&
+	git checkout master &&
+	git checkout other &&
+	git rebase master moodle &&
+	git checkout - &&
+	test "z$(git symbolic-ref HEAD)" = "zrefs/heads/master"
+'
+
+test_expect_failure '"checkout -" works after a rebase -i A' '
+	git checkout master &&
+	git checkout other &&
+	git rebase -i master &&
+	git checkout - &&
+	test "z$(git symbolic-ref HEAD)" = "zrefs/heads/master"
+'
+
+test_expect_failure '"checkout -" works after a rebase -i A B' '
+	git branch foodle master~1 &&
+	git checkout master &&
+	git checkout other &&
+	git rebase master foodle &&
+	git checkout - &&
+	test "z$(git symbolic-ref HEAD)" = "zrefs/heads/master"
+'
+
 test_done