graph: fix lack of color in horizontal lines

graph: fix lack of color in horizontal lines

In some cases, horizontal lines in rendered graphs can lose their
coloring. This is due to a use of graph_line_addch() instead of
graph_line_write_column(). Using a ternary operator to pick the
character is nice for compact code, but we actually need a column to
provide the color.

Add a test to t4215-log-skewed-merges.sh to prevent regression.

Reported-by: Jeff King <redacted>
Signed-off-by: Derrick Stolee <redacted>
Signed-off-by: Junio C Hamano <redacted>