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>

Author: szeder

t/t6120-describe.sh

@@ -1,15 +1,16 @@
 #!/bin/sh
 
-test_description='test describe
+test_description='test describe'
+
+#  o---o-----o----o----o-------o----x
+#       \   D,R   e           /
+#        \---o-------------o-'
+#         \  B            /
+#          `-o----o----o-'
+#                 A    c
+#
+# First parent of a merge commit is on the same line, second parent below.
 
-                       B
-        .--------------o----o----o----x
-       /                   /    /
- o----o----o----o----o----.    /
-       \        A    c        /
-        .------------o---o---o
-                   D,R   e
-'
 . ./test-lib.sh
 
 check_describe () {