Environment variables are global and hard to reason about.
Use the `--git-dir` and `--work-tree` arguments when invoking `git`
instead of relying on the environment.
Add a test to ensure that difftool's dir-diff feature works when these
variables are present in the environment.
Signed-off-by: David Aguilar <davvid@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>