linenoise: squelch some useless undo items.

Sometimes edits don't do anything, and so the undo item
recorded for them is superfluous. When undo is used
nothing appears to happen for a turn.

We use two strategies: detect the noop change
and avoid recording the undo (or doing anything),
or else detecting the noop undo afterward and
removing it from the undo stack.

* linenoise/linenoise.c (remove_noop_undo): New static
function.
(edit_delete, edit_backspace): Remove noop undo.
(edit_delete_prev_word): Don't record undo, move
memory or update vars if there is no word to delete.
(edit_in_editor): Remove noop undo.
(edit): For Ctrl-T, don't record undo or do anything
other than clear the selection if the cursor
is leftmost already.

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