Problem:
On windows, if a drive-relative path doesn't contain a slash,
`path_to_absolute` can't split out the relative component, causing
expansion to fails. e.g., `c:` `c:.` `c:..` `c:foo.md`
Solution:
For these cases, we can pass letter and colon to `path_full_dir_name`.
Notably, `..` is included as well.
if that relative path exists, it can be expanded correctly.
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