bug#56355: 29.0.50; Implement file-parent-directory

[daanturo]

About the comparator again, I hope that we use equal instead of file-equal-p.

The former gives far better performance.

Let my-parent-directory be a version of file-parent-directory with file-equal-p
replaced by equal, both are natively compiled.

```
(length recentf-list)
=> 659

(benchmark 1 '(mapcar 'my-parent-directory recentf-list))
=> "Elapsed time: 0.006954s"

(benchmark 1 '(mapcar 'file-parent-directory recentf-list))
=> "Elapsed time: 0.230073s"

```

That's about a 33-time difference in speed.

Also it's not like that (file-name-directory (directory-file-name <>)) is
supposed to resolve symbolic/hard links anyway.

-- 
Daanturo.