merge-recursive: flush output buffer even when erroring out

merge-recursive: flush output buffer even when erroring out

Ever since 66a155b (Enable output buffering in merge-recursive.,
2007-01-14), we had a problem: When the merge failed in a fatal way, all
regular output was swallowed because we called die() and did not get a
chance to drain the output buffers.

To fix this, several modifications were necessary:

- we needed to stop die()ing, to give callers a chance to do something
  when an error occurred (in this case, flush the output buffers),

- we needed to delay printing the error message so that the caller can
  print the buffered output before that, and

- we needed to make sure that the output buffers are flushed even when
  the return value indicates an error.

The first two changes were introduced through earlier commits in this
patch series, and this commit addresses the third one.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>