gitk: update x11 scrolling for TclTk 8.6 / TIP 171

gitk: update x11 scrolling for TclTk 8.6 / TIP 171

gitk has x11 mouse bindings that receive button presses, not MouseWheel
events, as this is the Tk implementation through Tk 8.6. On x11, gitk
translates each button event to a scrolling value of +/- 5 for the upper
three panes that scroll vertically as one unit. gitk applies similar
scaling for horizontal scaling of the lower-left commit details pane
(ctext), but not for vertical scrolling of either of the bottom panes.
Rather, the Tk default scrolling actions are used for vertical
scrolling.

Let's make X11 behave similarly to the just modified win32 platform. Do
so by connecting vertical and horizontal scrolling events for the same
items bound in 'proc bind_mousewheel' and using the same user preference
values.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>