The prefix_filename function used to do an early return when
there was no prefix on non-Windows platforms, but always
allocated on Windows so that it could call convert_slashes().
Now that the function always allocates, we can unify the
logic and make convert_slashes() the only conditional part.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>