Merge branch 'tr/diff-submodule-no-reuse-worktree' into maint

"git diff --external-diff" incorrectly fed the submodule directory
in the working tree to the external diff driver when it knew it is
the same as one of the versions being compared.

* tr/diff-submodule-no-reuse-worktree:
diff: do not reuse_worktree_file for submodules

diff --cc diff.c
Simple merge

diff --cc t/t4020-diff-external.sh
Simple merge