gitk: Improve behaviour of drop-down lists

gitk: Improve behaviour of drop-down lists

The drop-down lists used for things like the criteria for finding
commits (containing/touching paths/etc.) use a combobox if we are
using the ttk widgets.  By default the combobox exports its value
as the selection when it is changed, which is unnecessary, and sometimes
the combobox wouldn't release the selection, which is annoying.

To fix this, we make these comboboxes not export their selection,
and also clear their selection whenever they are changed.  This makes
them more like a simple selection of alternatives, improving the look
and feel of gitk.

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