bug#45764: icomplete in buffer completion does not work for completion-at-point

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

> `C-j' is bound to `icomplete-force-complete-and-exit', and it calls
> `minibuffer-force-complete-and-exit' unconditionally, which deletes the
> buffer contents.

There are 2 problems, both of which stem from the fact that the code has
not been adjusted to the case where it's working in a normal buffer
rather than in the minibuffer:
- it calls `minibuffer-exit` which is wrong for in-buffer completion
  (it should probably exit `completion-in-region-mode` instead).
- it assumes the area of completion is the whole buffer (it should
  probably use `icomplete--field-end/beg`).

> I can't get icomplete-in-buffer to work in either *shell* or in other
> modes -- has this feature gotten much testing/use?

`icomplete-in-buffer` needs more love, indeed.
[ BTW, I see its docstring says that it doesn't affect
  `dabbrev-completion` but that's not true any more.  ]


        Stefan