bug#12831: 24.2.50; diff-remove-trailing-whitespace is broken

[Chong Yidong]

The command diff-remove-trailing-whitespace, newly introduced for Emacs
24.3, is broken.  AFAICT, it can't easily be made to work right.

1) diff-remove-trailing-whitespace is confused about the order of files
   in the diff:

echo "a" > oldfile
echo "b  " > newfile
emacs -Q
M-x diff RET newfile RET oldfile RET
C-x o
M-x diff-remove-trailing-whitespace RET
  => no trailing whitespace fixes needed

   (Note that the new file is by convention the FIRST argument to M-x
   diff and the SECOND argument to the `diff' command.)  This can be
   fixed by replacing (diff-find-source-location t t) in the definition
   of diff-remove-trailing-whitespace.  But we hit the second problem:

2) After the trailing whitespaces are fixed in the underlying files, the
   trailing whitespace still remains in the *Diff* buffer.  The basic
   problem is that the algorithm of this command is too simplistic: it
   looks for trailing whitespace in the Diff buffer, jumps to the
   corresponding location given by diff-find-source-location, looks for
   trailing whitespace on that line, and deletes it.

   This simple algorithm can lead to serious mistakes.  There's no
   attempt to check if the point in the Diff buffer corresponds to a
   "before" or "after" line (nothing ought to be done for the former,
   but the existing command is happy to use it), or whether the
   "trailing whitespace" that it found is merely the space which is
   customarily inserted after "!" in context diffs.

Unless someone comes up with a version of this command that works right,
I think we should go back to the drawing board on this, and remove it
from Emacs 24.3.