The comment's continuation line starts with ";;;;".  I see two problems
with this:

1. The padding characters are not propertized, so we get two fontified
   semicolons, then two unfontified semicolons.

2. Visually, this looks sort of cluttered.  I have searched Debbugs and
   emacs-devel for a rationale for using (substring fcp -1) instead of
   unconditionally using spaces, but I could not find any.

Should we simplify adaptive-wrap-fill-context-prefix to stop bothering
with (substring fcp -1) and simply use " " for extra-indent?  If not,
should we at least propertize the padding characters, applying whatever
properties we find on (substring fcp -1)?

I can produce patches for either approach; I'd like to know what we'd
prefer first.


2. When wrapping lines that have an :extend t face
==================================================

See second screenshot, taken from emacs -Q -rv, with:

#+begin_src elisp
(progn
  (package-initialize)
  (global-visual-line-mode)
  (setq visual-line-fringe-indicators '(left-curly-arrow right-curly-arrow))
  (add-hook 'visual-line-mode-hook 'adaptive-wrap-prefix-mode)
  (setq-default adaptive-wrap-extra-indent 2))
#+end_src

(Ignore the fact that continuation lines are indented much further for
removed lines than for added lines, that's because adaptive-fill-regexp
matches -'s but not +'s.)