Fix panic in long item completion in ide menu

[HKalbasi]

Ide menu uses ... for items that are very long, and if the prompt already contains some string inside of ... it will panic. It can be reproduced by completing the largest item in the ide_completions, then deleting one or two characters, and hitting Tab again.

The panic was found in #821

[fdncred]

This is how I was able to repro the bug and test this fix.

1. cargo run --example ide_completions
2. h<tab>
3. the longest entry should now be selected with ... at the end
4. hit enter
5. hit backspace
6. hit tab

Thanks for fixing this. Let's go!