Even though an fix was attempted in Git 2.9.3 days, but running
"git difftool --dir-diff" from a subdirectory never worked. This
has been fixed.
* jk/difftool-in-subdir:
difftool: rename variables for consistency
difftool: chdir as early as possible
difftool: sanitize $workdir as early as possible
difftool: fix dir-diff index creation when in a subdirectory