Unnecessary completion request when updating the common prefix to the command line.

[blindFS]

From nushell/nushell#16860, we can tell that there's something to optimize.

Steps to reproduce

def completion [] {
  ['"first item"' '"second item"']
}

export def foo [c: string@completion] {
  print $c
}

1. foo <tab> returns those 2 items with the common prefix "
2. Reedline updates the line text to foo "
3. During menu redraw (maybe?), the completion results are updated to those for the new text foo ", which is totally unnecessary in theory.

It could potentially be a dealbreaker for time costing external completer.

[ysthakur]

This probably isn't something we want to do right now, but in the future, we should offer a convenient way to cache results.

[blindFS]

This probably isn't something we want to do right now, but in the future, we should offer a convenient way to cache results.

Maybe unrelated, but this reminds me of #881.

I will try fixing that one first, hopefully it will make fixing this much easier.