gitk highlighting descendents/ancestors

I have implemented a feature in gitk (on the "new" branch) where it
can highlight the commits that are, or are not, descendents or
ancestors of the selected commit.  For now it is invoked via a
drop-down menu.  Does this look useful to people?

At the moment, if you select "Not descendent" then the selected
commit and all the commits below it will be highlighted (since they
can't be descendents of the selected commit).  Similarly if you select
"Not ancestor" then the selected commit and all commits above it are
highlighted.  That's technically correct but doesn't seem very useful;
maybe I should suppress those highlights.

Also, whatever matches the string put in the "Find" field will be
highlighted.  I have taken out the "Files" and "Pickaxe" options in
the Find function, and put the pickaxe function into a new drop-down
menu for the highlighting function.  I have code ready to go for
shift-up and shift-down to move to the previous/next highlighted row,
once Junio applies my tiny patch to builtin-diff-tree.c.

Paul.