gitk: Eliminate diagonal arrows

This changes the optimizer to insert pads to straighten downward
pointing arrows so they point straight down.  When drawing the parent
link to the first child in drawlineseg, this draws it with 3 segments
like other parent links if it is only one row high with an arrow.
These two things mean we can dispense with the workarounds for arrows
on diagonal segments.  This also fixes a couple of other minor bugs.

Signed-off-by: Paul Mackerras <paulus@samba.org>

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