Merge branch 'pm/gitk-update'

"gitk" update.

* pm/gitk-update:
  gitk: Resize panes correctly when reducing window size
  gitk: replace tabs with spaces
  gitk: fix the context menu not appearing in the presence of submodule diffs
  gitk: Un-hide selection in areas with non-default background color
  gitk: add diff lines background colors
  gitk: be prepared to be run in a bare repository
  gitk: Preserve window dimensions on exit when not using ttk themes
  gitk: don't highlight files after submodules as submodules
  gitk: fix branch name encoding error
  gitk: rename "commit summary" to "commit reference"