mergeview: capture < and > on marker lines.

mergeview: capture < and > on marker lines.

When cursor is on a marker line, capture < and > for manipulating the
merge.

On the first marker: lines are moved between top of orig and preceding
content.
On the second marker: lines are moved between bottom of orig and
following content.

I don't know what I want on other marker lines.

Signed-off-by: NeilBrown <neil@brown.name>