bug#15362: 24.1; pcomplete unnecessarily restricts tab-completion entry points

[Vivek Dasmohapatra]

> So if cycling is to take place, it should presumably not be done by
> pcomplete but by completion-at-point (e.g. obeying
> completion-cycle-threshold).
>
> IOW, I don't understand how your patch can fix your problem.
> Can you provide a reproducible recipe starting from "emacs -Q"?

  emacs -Q

  customise the erc group, make sure erc-pcomplete is on (it
  should be)

  customise erc-complete-functions, add erc-pcomplete to
  the head of that list

  M-x erc RET … ; connect to freenode, enter #emacs

  j TAB ; big list of nicks beginning with jpops up

  l TAB ; unconditionally completes to whichever jl… nick
        ; was at the head of the list.

Analaysis:

  TAB                           → completion-at-point
  completion-at-point           → completion-at-point-functions
  completion-at-point-functions → erc-complete-word-at-point
  erc-complete-word-at-point    →
      (run-hook-with-args-until-success 'erc-complete-functions)
  erc-complete-functions        → erc-pcomplete
  erc-pcomplete                 → (call-interactively 'pcomplete)
  pcomplete                     →
      (memq last-command '(pcomplete
                           pcomplete-expand-and-complete
                           pcomplete-reverse)

  The above test controls entry into the cycling code.