t6120-describe: correct test repo history graph in comment

t6120-describe: correct test repo history graph in comment

At the top of 't6120-describe.sh' an ASCII graph illustrates the
repository's history used in this test script.  This graph is a bit
misleading, because it swapped the second merge commit's first and
second parents.

When describing/naming a commit it does make a difference which parent
is the first and which is the second/Nth, so update this graph to
accurately represent that second merge.

While at it, move this history graph from the 'test_description'
variable to a regular comment.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>