gitk: Fix a bug in drawing the selected line as a thick line

gitk: Fix a bug in drawing the selected line as a thick line

If you clicked on a line, so that it was drawn double-thickness,
and then scrolled to bring on-screen a child that hadn't previously
been drawn, the lines from it to the selected line were drawn
single-thickness.  This fixes it so they are drawn double-thickness.
This also removes an unnecessary setting of phase in drawrest.

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