* lisp/vc/diff-mode.el: Improve diff-font-lock-prettify

* lisp/vc/diff-mode.el: Improve diff-font-lock-prettify

A few tweaks to the previous code for corner case problems, and a new
feature, which is to move the +/- signs to the left fringe.

(diff--font-lock-cleanup, diff--filter-substring): New functions.
(diff-mode): Use them.
(diff--font-lock-refined): Mark the overall overlays as `diff-mode
fine` as well, so they get properly cleaned up when changing mode.
(diff-fringe-add, diff-fringe-del, diff-fringe-rep, diff-fringe-nul):
New bitmaps.
(diff--font-lock-prettify): Move the +/- signs to the fringe.
(diff-wiggle): Use 'user-error'.