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

From https://github.com/nushell/nushell/issues/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.

Start by reproducing the Nushell completion example and trace Reedline's completion and menu-redraw flow after the common prefix changes. Identify where the updated line triggers another completion request, then verify that the existing results are not requested again when the prefix update does not require new results.