[merge addon] Shows differences when the non-edit pane is updated

Previous behaviour was to only update the displayed differences if
the edit pane was modified.  There exists use-cases such that the
differences should be updated when any of the panes change.

I don't expect this to introduce any performance regressions due
to the existing debounce code within the update function.

Signed-off-by: ciaranj <ciaranj@gmail.com>