t3200: prepare for `main` being shorter than `master`

In the test case adjusted by this patch, we want to cut just after the
longest shown ref name. Since `main` is shorter than `master`, we need
to decrease the number of characters. Since `topic` is shown, too, and
since that is only one character shorter than `master`, we decrement the
length by one instead of two.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3200-branch.sh

@@ -375,9 +375,9 @@ test_expect_success 'git branch --column -v should fail' '
 	test_must_fail git branch --column -v
 '
 
-test_expect_success 'git branch -v with column.ui ignored' '
+test_expect_success PREPARE_FOR_MAIN_BRANCH 'git branch -v with column.ui ignored' '
 	git config column.ui column &&
-	COLUMNS=80 git branch -v | cut -c -9 | sed "s/ *$//" >actual &&
+	COLUMNS=80 git branch -v | cut -c -8 | sed "s/ *$//" >actual &&
 	git config --unset column.ui &&
 	cat >expect <<\EOF &&
   a/b/c