gitk: Resize panes correctly when reducing window size

The resizeclistpanes and resizecdetpanes procedures attempt to keep the horizontal proportions of the panes of the gitk window approximately constant when the gitk window is resized. However, if the size is reduced enough that an existing sash position would go outside the window, Tk moves the sash to the left to keep it inside the window (without moving other sash positions to keep the proportions). This happens before these resize procedures get control, and so they work with incorrect proportions. To fix this, we record the sash positions we set previously and use those previously-set sash positions rather than the current sash positions when computing the proportions. Signed-off-by: Paul Mackerras <paulus@ozlabs.org>