prompts deduplicate their history

[Axlefublr]

Normally, if you execute these three command mode commands in a row:

:echo banana
:echo potato
:echo banana

You will have this in your command mode history (meaning, : register)

echo banana
echo potato
echo banana

If we scale this up to a long helix session, you might end up having a lot of repeats, that are annoying to ↑ through.

With this pr, when you accept your input in a prompt and it's added to history, all the previous times this exact input was in history, is cleaned up.
So, you'll end up with just this instead:

echo potato
echo banana

If you :echo potato from this state, now your history is:

echo banana
echo potato

I use command mode as the example, but all prompts that have history are affected by this improvement.

If you like this change, check out my #15266 also.