gitk: Improve the drawing of links to parent lines

gitk: Improve the drawing of links to parent lines

The way gitk used to draw the lines joining a commit to the lines
representing its parents was sometimes visually ambiguous, especially
when the line to the parent had a corner that coincided with a corner
on another line.

This improves things by using a smaller slanting section on the line
joining a commit to a parent line if the parent line is vertical where
it joins on.  It also optimizes the drawing a little in the case where
the parent line slants towards this commit already.

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