rebase: refuse to switch to a branch already checked out elsewhere (test)

In b5cabb4a9 (rebase: refuse to switch to branch already checked out
elsewhere, 2020-02-23) we add a condition to prevent a rebase operation
involving a switch to a branch that is already checked out in another
worktree.

A bug has recently been fixed that caused this to not work as expected.

Let's add a test to notice if this changes in the future.

Signed-off-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3400-rebase.sh

@@ -388,6 +388,20 @@ test_expect_success 'switch to branch checked out here' '
 	git rebase main main
 '
 
+test_expect_success 'switch to branch checked out elsewhere fails' '
+	test_when_finished "
+		git worktree remove wt1 &&
+		git worktree remove wt2 &&
+		git branch -d shared
+	" &&
+	git worktree add wt1 -b shared &&
+	git worktree add wt2 -f shared &&
+	# we test in both worktrees to ensure that works
+	# as expected with "first" and "next" worktrees
+	test_must_fail git -C wt1 rebase shared shared &&
+	test_must_fail git -C wt2 rebase shared shared
+'
+
 test_expect_success 'switch to branch not checked out' '
 	git checkout main &&
 	git branch other &&