Merge remote-tracking branch 'paulus/master' into pm/gitk-update

* paulus/master: 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"