Merge branch 'pt/gitgui-updates'

Merge branch 'pt/gitgui-updates'

A new version of git-gui, now at its 0.21.0 tag.

* pt/gitgui-updates: (22 commits)
  git-gui: set version 0.21
  git-gui: Mark 'All' in remote.tcl for translation
  git-gui i18n: Updated Bulgarian translation (565,0f,0u)
  git-gui: avoid persisting modified author identity
  git-gui: handle the encoding of Git's output correctly
  git-gui: unicode file name support on windows
  git-gui: Update Russian translation
  git-gui: maintain backwards compatibility for merge syntax
  git-gui i18n: mark string in lib/error.tcl for translation
  git-gui: fix incorrect use of Tcl append command
  git-gui i18n: mark "usage:" strings for translation
  git-gui i18n: internationalize use of colon punctuation
  git-gui: ensure the file in the diff pane is in the list of selected files
  git-gui: support for $FILENAMES in tool definitions
  git-gui: fix initial git gui message encoding
  git-gui/po/glossary/txt-to-pot.sh: use the $( ... ) construct for command substitution
  git-gui (Windows): use git-gui.exe in `Create Desktop Shortcut`
  git-gui: fix detection of Cygwin
  Amend tab ordering and text widget border and highlighting.
  Allow keyboard control to work in the staging widgets.
  ...