mv: let 'git mv file no-such-dir/' error out on Windows, too

The previous commit c57f628 (mv: let 'git mv file no-such-dir/' error out)
relies on that rename("src", "dst/") fails if directory dst does not
exist (note the trailing slash). This does not work as expected on Windows:
This rename() call is successful. Insert an explicit check for this case.

This changes the error message from

   $ git mv file no-such-dir/
   fatal: renaming 'file' failed: Not a directory

to

   $ git mv file no-such-dir/
   fatal: destination directory does not exist, source=file, destination=no-such-dir/

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>