merge-recursive: eliminate flush_buffer() in favor of write_in_full()
flush_buffer() is a thin wrapper around write_in_full() with two very
confusing properties:
* It runs a loop to handle short reads, ensuring that we write
everything. But that is precisely what write_in_full() does!
* It checks for a return value of 0 from write_in_full(), which cannot
happen: it returns this value only if count=0, but flush_buffer()
will never call write_in_full() in this case.
Remove it.
Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>