bug#27985: [25.2] delete-indentation: restore correct point (and..) [PATCH INCLUDED]

[Boruch Baum]

GNU Emacs 25.2.2 (x86_64-pc-linux-gnu, GTK+ Version 3.22.16)
of 2017-07-11, modified by Debian

1] Function `delete-indentation' does not preserve point. The attached
patch fixes that for joining lines both forward and backward.

2] Also included is a wrapper function, `my-join-lines' that I'd like to
propose be included in emacs.

2.1] The wrapper function adds the ability to:

2.1.1] Act on regions;

2..1.2] Accept a numeric prefix argument for the number of lines to join
in either direction.

2.2] Lines of the wrapper function annotated on the right with the
string `; ┃' are only necessary if the proposed patch to function
`delete-indentation' is rejected.

-- 
hkp://keys.gnupg.net
CA45 09B5 5351 7C11 A9D1  7286 0036 9E45 1595 8BC0

(defun delete-indentation (&optional arg)
  "Join this line to previous and fix up whitespace at join.
If there is a fill prefix, delete it from the beginning of this line.
With argument, join this line to following line."
  (interactive "*P")
  (let ((return-point (point))
        (return-col (if (not arg) (- (point-at-eol) (point)))))
   (beginning-of-line)
   (if arg (forward-line 1))
   (if (eq (preceding-char) ?\n)
       (progn
        (delete-region (point) (1- (point)))
        ;; If the second line started with the fill prefix,
        ;; delete the prefix.
        (if (and fill-prefix
                 (<= (+ (point) (length fill-prefix)) (point-max))
                 (string= fill-prefix
                          (buffer-substring (point)
                                            (+ (point) (length fill-prefix)))))
            (delete-region (point) (+ (point) (length fill-prefix))))
        (fixup-whitespace)
    (if return-col
      (goto-char (- (point-at-eol) return-col))
     (goto-char return-point))))))

(defun my-join-lines (&optional arg)
  "Join multiple lines, forward or backwards.

By default, joins forward one line (the standard `join-lines',
an alias for `delete-indentation', and bound by default to M-^,
joins backward by default).

With a prefix argument joins that number of lines in +/- direction.

With a region selected, joins all lines in the region."
  (interactive "p")
  (let ((return-point (point))                           ; ┃
        return-col)                                      ; ┃
    (if (use-region-p)
      (let ((begin (region-beginning)))
        (when (= (point) (region-end))                   ; ┃
          (setq return-col (- (point-at-eol) (point))))  ; ┃
        (goto-char (region-end))
        (while (> (point-at-bol) begin)
          (join-line)))
     (if arg
       (if (> arg 0)
         (dotimes (i arg) (join-line t))
        (setq return-col (- (point-at-eol) (point)))     ; ┃
        (dotimes (i (- 0 arg)) (join-line)))
      (join-line t)))
    (if return-col                                       ; ┃
      (goto-char (- (point-at-eol) return-col))          ; ┃
     (goto-char return-point))))                         ; ┃