builtin-mv.c: check for unversionned files before looking at the destination.

builtin-mv.c: check for unversionned files before looking at the destination.

The previous code was failing in the case where one moves an
unversionned file to an existing destination, with mv -f: the
"existing destination" was checked first, and the error was cancelled
by the force flag.

We now check the unrecoverable error first, which fixes the bug.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>