Use the open(2) flag O_EXCL to ensure the file doesn't already exist
instead of (racily) calling stat(2) through file_exists(). While at it
switch to xopen() to reduce code duplication and get more consistent
error messages.
Signed-off-by: Rene Scharfe <l.s.r@web.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>