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

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>