summarize: fix collapse all

[rupran]

With the current implementation, 'collapse all' (shortcut 'H')
crashes with the following error when anything other than the
top-level item is selected (for this particular key, the first
sub-item was selected):

Traceback (most recent call last):
  File "./summarize.py", line 753, in <module>
    draw_infobox(gui, lookup[textbox.selected_index])
KeyError: 1

Fix it by scrolling to the top whenever collapsing the whole tree.