t3404: begin "exchange commits with -p" test with correct preconditions

The test case shows a bug in 'rebase -p', but even if the bug were fixed
the test would fail because it did not ensure that the preconditions match
the postconditions that were checked. Insert the suitable 'git checkout'.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Acked-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3404-rebase-interactive.sh

@@ -247,6 +247,7 @@ test_expect_success '-p handles "no changes" gracefully' '
 '
 
 test_expect_failure 'exchange two commits with -p' '
+	git checkout H &&
 	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)