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