mv: fails if source dir name is subset of target dir name

This issue was mentioned by @allaboutevemirolive in https://github.com/uutils/coreutils/issues/4833#issuecomment-1786554523:
```
$ mkdir test test2
$ cargo run mv test test2
mv: cannot move 'test' to a subdirectory of itself, 'test2/test'
```
Using different directory names, the same steps work fine:
```
$ mkdir aa bb
$ cargo run mv aa bb
$ ls bb
aa
```