bug#21710: 25.0.50; self-insert-command before a folded outline inserts after the outline

[Oleh Krehel]

To reproduce with "emacs -Q", create a file ~/outline-test with the
following contents:

* foo
some text
* bar
some text

Open the file, move to start, "M-x" `outline-minor-mode',
"M-x" `outline-hide-sublevels'.

Move to the end of bar and insert "tt" - it works correctly.

But move to the end of foo and insert "tt": here's what it looks like
("|" is the point):

* foo...t|
* bartt...

Here's the full text after "M-x" `outline-show-all':

* foot
some textt
* bartt
some text

As you see, the second "t" was inserted after the outline.
Since Org mode uses outlines, this bug also appears there.