Proposal: diff-remove-trailing-blanks

["Óscar Fuentes" <ofv@wanadoo.es>]

Some time ago Steffan suggested that this function could be a worth
addition to diff-mode. If this is ok, I need to sign the copyright
papers.

(defun diff-remove-trailing-blanks ()
  "When on a buffer that contains a diff, inspects the
differences and removes trailing whitespace (spaces, tabs) from
the lines modified or introduced by this diff. Shows a message
with the name of the altered buffers, which are unsaved.  If a
file referenced on the diff has no buffer and needs to be fixed,
a buffer visiting that file is created."
  (interactive)
  (goto-char (point-min))
  ;; We assume that the diff header has no trailing whitespace.
  (setq modified-buffers nil)
  (setq white-positions nil)
  (while (re-search-forward "^[+!>].*[ \t]+$" (point-max) t)
    (save-excursion
      (destructuring-bind (buf line-offset pos src dst &optional switched)
	  (diff-find-source-location t t)
	(when line-offset
	  (set-buffer buf)
	  (save-excursion
	    (goto-char (+ (car pos) (cdr src)))
	    (beginning-of-line)
	    (when (re-search-forward "\\([ \t]+\\)$" (line-end-position) t)
	      (when (not (member buf modified-buffers))
		(push buf modified-buffers))
	      (goto-char (match-end 0))
	      (push (point-marker) white-positions)
	      (goto-char (match-beginning 0))
	      (push (point-marker) white-positions)
	      (push buf white-positions)))))))
  (while white-positions
    (save-excursion
      (set-buffer (pop white-positions))
      (delete-region (pop white-positions) (pop white-positions))))
  (if modified-buffers
      (let ((msg "Deleted new trailing whitespace from:"))
	(dolist (f modified-buffers)
	  (setq msg (concat msg " `" (buffer-name f) "'")))
	(message "%s" msg))
    (message "No fixes needed.")))