Merge branch 'jk/difftool-in-subdir' into maint

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