gitk: Improve appearance of first child links

The point where the line for a parent joins to the first child
shown is visually different from the lines to the other children,
because the line doesn't branch, but terminates at the child.
Because of this, we now treat the first child a little differently
in the optimizer, and we draw its link in drawlineseg rather
than drawparentlinks.  This improves the appearance of the graph.

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