graph: add test to demonstrate horizontal line bug

A previous test in t4215-log-skewed-merges.sh was added to demonstrate
exactly the topology of a reported failure in "git log --graph". While
investigating the fix, we realized that multiple edges that could
collapse with horizontal lines were not doing so.

Specifically, examine this section of the graph:

	| | | | | | *
	| |_|_|_|_|/|\
	|/| | | | |/ /
	| | | | |/| /
	| | | |/| |/
	| | |/| |/|
	| |/| |/| |
	| | |/| | |
	| | * | | |

Document this behavior with a test. This behavior is new, as the
behavior in v2.24.1 has the following output:

	| | | | | | *-.
	| | | | | | |\ \
	| |_|_|_|_|/ / /
	|/| | | | | / /
	| | |_|_|_|/ /
	| |/| | | | /
	| | | |_|_|/
	| | |/| | |
	| | * | | |

The behavior changed logically in 479db18b ("graph: smooth appearance
of collapsing edges on commit lines", 2019-10-15), but was actually
broken due to an assert() bug in 458152cc ("graph: example of graph
output that can be simplified", 2019-10-15). A future change could
modify this behavior to do the following instead:

	| | | | | | *
	| |_|_|_|_|/|\
	|/| | | | |/ /
	| | |_|_|/| /
	| |/| | | |/
	| | | |_|/|
	| | |/| | |
	| | * | | |

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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