merge-recursive: write the commit title in one go

In 66a155b (Enable output buffering in merge-recursive., 2007-01-14), we changed the code such that it prints the output in one go, to avoid interfering with the progress output. Let's make sure that the same holds true when outputting the commit title: previously, we used several printf() statements to stdout and assumed that stdout's buffer is large enough to hold the entire commit title. Apart from making that speculation unnecessary, we change the code to add the message to the output buffer before flushing for another reason: the next commit will introduce a new level of output buffering, where the caller can request the output not to be flushed, but to be retained for further processing. This latter feature will be needed when teaching the sequencer to do rebase -i's brunt work: it wants to control the output of the cherry-picks (i.e. recursive merges). Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>