Autocomplete for usernames should handle punctuation better

Right now when you try to @-mention someone, and then put punctuation for example or a comma or a period, it auto-completes in a very frustrating way.

To reproduce, do the following:

1. Autocomplete someones username in an issue (using tab, for example)
2. Autocomplete automatically puts a space, which you don't want. So backspace it off.
3. Now autocomplete pops up again, and type in a period .
4. Autocomplete updates its recommendations to show the generic list, for example @all, since it now matches nothing.
5. If you were to autocomplete again, it would give you the wrong autocomplete

This is a frustrating experience personally. My console-derived habit of tab-completing everything makes my initial reaction to tab-complete after the period incorrect. I instead have to re-train myself to use spacebar, which does not cause the incorrect autocomplete to apply.

I wonder if we can figure out a better way to approach this:

1. If there were valid autocomplete selections before, and now nothing matches, don't show the "all list". Perhaps just show nothing.
2. Maybe something else I can't think of right now =)