Merge branch 'jk/difftool-in-subdir'

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
