gitk: Prevent parent link from overwriting commit headline

gitk: Prevent parent link from overwriting commit headline

When I made drawlineseg responsible for drawing the link to the first
child rather than drawparentlinks, that meant that the right-most X
value computed by drawparentlinks didn't include those first-child
links, and thus the first-child link could go over the top of the
commit headline.  This fixes it.

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