t3404: check root commit in 'rebase -i --root reword root commit'

When testing a reworded root commit, ensure that the squash-onto commit
which is created and amended is still the root commit.

Suggested-by: Phillip Wood <phillip.wood@talktalk.net>
Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Todd Zullinger <tmz@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3404-rebase-interactive.sh

@@ -977,7 +977,8 @@ test_expect_success 'rebase -i --root reword root commit' '
 	set_fake_editor &&
 	FAKE_LINES="reword 1 2" FAKE_COMMIT_MESSAGE="A changed" \
 	git rebase -i --root &&
-	git show HEAD^ | grep "A changed"
+	git show HEAD^ | grep "A changed" &&
+	test -z "$(git show -s --format=%p HEAD^)"
 '
 test_expect_success C_LOCALE_OUTPUT 'rebase --edit-todo does not work on non-interactive rebase' '