Enable output buffering in merge-recursive.

Enable output buffering in merge-recursive.

Buffering all message output until a merge invocation is complete is
necessary to prevent intereferring with a progress meter that would
indicate the number of files completely merged, and how many remain.
This change does not introduce a progress meter, but merely lays
the groundwork to buffer the output.

To aid debugging output buffering is only enabled if verbosity
is lower than 5.  When using verbosity levels above 5 the user is
probably debugging the merge program itself and does not want to
see the output delayed, especially if they are stepping through
portions of the code in a debugger.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>