[Orgmode] auto-fill bug

[Adam Spiers]

I just found some strange behaviour with auto-fill mode on:

--------- 8< --------- 8< --------- 8< --------- 8< --------- 8< ---------
#+TITLE: test title

* First heading
  - Turn this into a long sentence to see the bug.

--------- 8< --------- 8< --------- 8< --------- 8< --------- 8< ---------

If you extend the sentence by adding only words and spaces, when it
wraps, indentation is broken and a # appears from nowhere:

--------- 8< --------- 8< --------- 8< --------- 8< --------- 8< ---------
#+TITLE: test title

* First heading
  - Turn this into a long sentence to see the bug.  Or you can start a
#new sentence; either way you get that strange #-prefixed outdentation.
--------- 8< --------- 8< --------- 8< --------- 8< --------- 8< ---------

The odd thing is that if there is nothing in the buffer after the
point when you start (not even a newline) then the bug vanishes.

Reproduced with a recent git clone claiming to be 5.20, and also with
5.17.  Thanks!