Sync with master

* master:
  Third batch
  git-gui: add hotkey to toggle "Amend Last Commit"
  git-gui: add horizontal scrollbar to commit buffer
  git-gui: convert new/amend commit radiobutton to checkbutton
  git-gui: add hotkeys to set widget focus
  git-gui: allow undoing last revert
  git-gui: return early when patch fails to apply
  git-gui: allow reverting selected hunk
  git-gui: allow reverting selected lines