Merge branch 'ml/abandon-old-version' (early part)

* 'ml/abandon-old-version' (early part): gitk: allow horizontal commit-graph scrolling gitk: update aqua scrolling for TclTk 8.6 / TIP171 gitk: update x11 scrolling for TclTk 8.6 / TIP 171 gitk: update win32 scrolling for Tk 8.6 / TIP 171 gitk: mousewheel scrolling functions for Tk 8.6 gitk: wheel scrolling multiplier preference gitk: separate x11 / win32 / aqua Mouse bindings gitk: remove non-ttk support code gitk: replace ${NS} with ttk gitk: always use themed Tk (ttk) gitk: use $config_variables as list for save/restore gitk: remove implementations for Tcl/Tk < 8.6 gitk: Make TclTk 8.6 the minimum, allow 8.7 gitk: remove code targeting git <= 1.7.2 gitk: require git >= 2.20