Do a cross-project merge of Paul Mackerras' gitk visualizer

gitk is really quite incredibly cool, and is great for visualizing what
is going on in a git repository.  It's especially useful when you are
looking at what has changed since a particular version, since it
gracefully handles partial trees (and this also avoids the expense of
looking at _all_ changes in a big project).

For example, to see what changed in a merge after a "git pull", do

	gitk ORIG_HEAD..

to see only the new things.  Or you can simply do "gitk v2.6.12.." to
see what has changed since the v2.6.12 tag etc.

This merge itself is pretty interesting too, since it shows off a
feature of git itself that is incredibly cool: you can merge a
_separate_ git project into another git project.  Not only does this
keep all the history of the original project, it also makes it possible
to continue to merge with the original project and the union of the two
projects.

I don't think anybody else can do that.