notes-merge: drop dead zero-write code

notes-merge: drop dead zero-write code

We call write_in_full() with a size that we know is greater
than zero. The return value can never be zero, then, since
write_in_full() converts such a failed write() into ENOSPC
and returns -1.  We can just drop this branch of the error
handling entirely.

Suggested-by: Jonathan Nieder <redacted>
Signed-off-by: Jeff King <redacted>
Reviewed-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>