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