Merge branch 'da/difftool'

* da/difftool:
  difftool: Update copyright notices to list each year separately
  difftool: Use eval to expand '--extcmd' expressions
  difftool: Add '-x' and as an alias for '--extcmd'
  t7800-difftool.sh: Simplify the --extcmd test
  git-diff.txt: Link to git-difftool
  difftool: Allow specifying unconfigured commands with --extcmd
  difftool--helper: Remove use of the GIT_MERGE_TOOL variable
  difftool--helper: Update copyright and remove distracting comments
  git-difftool: Add '--gui' for selecting a GUI tool
  t7800-difftool: Set a bogus tool for use by tests