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"