write_entry(): use fstat() instead of lstat() when file is open

write_entry(): use fstat() instead of lstat() when file is open

Currently inside write_entry() we do an lstat(path, &st) call on a
file which have just been opened inside the exact same function.  It
should be better to call fstat(fd, &st) on the file while it is open,
and it should be at least as fast as the lstat() method.

Signed-off-by: Kjetil Barvik <barvik@broadpark.no>
Signed-off-by: Junio C Hamano <gitster@pobox.com>