* 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