gitk: Basic support for highlighting one view within another

With this, one view can be used as a highlight for another, so that
the commits that are in the highlight view are displayed in bold.
This required some fairly major changes to how the list of ids,
parents, children, and id to row mapping were stored for each view.
We can now be reading in several views at once; for all except the
current view, we just update the displayorder and the lists of parents
and children for the view.

This also creates a little bit of infrastructure for handling the
watch cursor.

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