gitk startup time improvement

In the 'dev' branch of the gitk.git repository at

git://git.kernel.org/pub/scm/gitk/gitk.git

there is now a version of gitk that is much faster at starting up than
the standard version.  Firstly, it doesn't lay out the whole graph,
only the parts that get displayed (plus a little bit either side), and
secondly, it caches the topology information that is used to determine
which tags a commit precedes and follows, and which branches it is on.

I'd like people to try it.  Comments, bug reports, patches etc. are
welcome, of course.

Paul.