Merge branch 'master' of https://github.com/j6t/gitk

Merge branch 'master' of https://github.com/j6t/gitk

* 'master' of https://github.com/j6t/gitk: (21 commits)
  gitk: remove header of now empty section "General options"
  gitk: separate upstream refs when using the sort-by-type option
  gitk: make 'sort-refs-by-type' optional and persistent
  gitk: sort by ref type on the 'tags and heads' view
  gitk: choosefont - remove a stray debugging line
  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
  ...