problem:
the variable names: `dir`, `name`, and `filename` leads to confusion if they are
the old or new names
solution:
add the prefix `old-` to match `new-`,
use (`old-` `new-`) + `short-name` for just the `file.ext` (the `short-` idea
came from the `spacemacs/rename-file` function that's defined above this one),
and `old-full-path` instead of `old-filename` makes it clear that it's not just
the files name and extension.
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