add-completion-to-headadd-completion-to-head is a compiled Lisp function in `
If COMPLETION-STRING is not in the database, add it to prefix list.
We add COMPLETION-STRING to the head of the appropriate prefix list,
or to the head of the list.
COMPLETION-STRING must be longer than `completion-prefix-min-length'.
Updates the saved string with the supplied string.
This must be very fast.
Returns the completion entry.