bug#34988: mv: check before asking users useless questions

[積丹尼 Dan Jacobson]

$ mv a b
mv: overwrite 'b'? y
mv: cannot overwrite non-directory 'b' with directory 'a'

User thinks well why didn't you check before uselessly asking me?

Full story:
$ mkdir a
$ touch b
mv: overwrite 'b'? y
mv: cannot overwrite non-directory 'b' with directory 'a'
$ \mv a b
mv: cannot overwrite non-directory 'b' with directory 'a'
$ alias
alias mv='mv -i'

So even if -i is in effect, please do the check before asking users
useless questions.