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>