Tab clobbers input when completions' common prefix is shorter than input string

Example is you are doing fuzzy completion, with `drop` completed to `Text.drop` and `Sequence.drop`. In this case, the common prefix of the completions is the empty string, and the user's input of `drop` is deleted (and no suggestions get printed).

Suggested behavior: Don't replace the user input if the common prefix is shorter than the input.

Another possible idea: somehow give the user more control over what happens to the line when completions are chosen (so they can override this behavior).