difftool: avoid $GIT_DIR and $GIT_WORK_TREE

difftool: avoid $GIT_DIR and $GIT_WORK_TREE

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>