Stop emojis from destroying your message

This is easy to reproduce in the following scenario with a bunch of half-completed emojis: ":a :a :a :a". Trying to complete anything but the last one starts replacing parts of the message because it only considers the last colon to the current completion identifier.

This change fixes that and said scenario can no longer cause a message massacare. This bug doesn't seem to affect the other completions because their searching in the string was correct, but I made sure they all share the same index now.

BUG: 479587 FIXED-IN: 24.12.3