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Use go_back_chunks() and go_forward_chunks() to move from the row
current_y is on to the row mouse_row is on.  Now softwrap mode and
non-softwrap mode will behave the same way when we can scroll edittop
partially off the screen, which is forthcoming.  Accordingly, remove
the call to ensure_line_is_visible(), as it no longer applies.

The old code did work, but it behaved differently between softwrap mode
(which counted down from edittop) and non-softwrap mode (which counted
up or down from current_y to take less time, and used a double loop to
keep current from going to NULL).

The new code counts up or down from current_y in both softwrap mode and
non-softwrap mode.  In non-softwrap mode, it also avoids the double loop,
since go_back_chunks() and go_forward_chunks() keep the filestructs they
operate on from going to NULL.