merge-ort: support having merge verbosity be set to 0

merge-ort: support having merge verbosity be set to 0

Various callers such as am & checkout set the merge verbosity to 0 to
avoid having conflict messages printed.  While this could be achieved by
avoiding the wrappers from merge-ort-wrappers and instead passing 0 for
display_update_msgs to merge_switch_to_result(), for simplicity of
converting callers simply allow them to also achieve this with the
merge-ort-wrappers by setting verbosity to 0.

Signed-off-by: Elijah Newren <redacted>
Reviewed-by: Taylor Blau <redacted>
Signed-off-by: Junio C Hamano <redacted>