"git restore --source" (without --staged) could create new files
(i.e. not present in index) on worktree to match the given source. But
the new files are not tracked, so both "git diff" and "git diff
<source>" ignore new files. "git commit -a" will not recreate a commit
exactly as the given source either.

Add --intent-to-add to help track new files in this case, which is the
default on the least surprise principle.