t3200: cleanup cruft of a test

Avoiding the clean up step of tests may help in some cases but in other
cases they cause the other unrelated tests to fail for unobvious reasons.
It's better to cleanup a few things to keep other tests from failing
as a result of it.

So, cleanup a cruft left behind by an old test in order for the changes that
are to be introduced to be independent of it.

Signed-off-by: Kaartic Sivaraam <kaarticsivaraam91196@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3200-branch.sh

@@ -559,6 +559,7 @@ test_expect_success 'use --set-upstream-to modify HEAD' '
 test_expect_success 'use --set-upstream-to modify a particular branch' '
 	git branch my13 &&
 	git branch --set-upstream-to master my13 &&
+	test_when_finished "git branch --unset-upstream my13" &&
 	test "$(git config branch.my13.remote)" = "." &&
 	test "$(git config branch.my13.merge)" = "refs/heads/master"
 '