gitk: New improved gitk

This is a new version of gitk which is much faster and has much better
graph layout.  It achieves the speed by only drawing the parts of the
canvases that are actually visible.  It also draws the commits in the
order that git-rev-list produces them, so if you use -d, you need to
have a recent enough git-rev-list that understands the --date-order
flag.

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