Save position in all windows with buffer

Originally, this logic only saved the position in the current window
for the buffer. If the same buffer is open in multiple windows, all
the other windows would jump to the beginning of the buffer. This
can be rather distracting when working on a complex module.

With this change, the buffer position is saved and restored for all
windows with the affected buffer.

Fix #49

Author: newt0311

blacken.el

@@ -144,8 +144,10 @@ Return black process the exit code."
 Show black output, if black exit abnormally and DISPLAY is t."
   (interactive (list t))
   (let* ((original-buffer (current-buffer))
-         (original-point (point))
-         (original-window-pos (window-start))
+         (original-window-states (mapcar
+                                  (lambda (w)
+                                    (list w (window-point w) (window-start w)))
+                                  (get-buffer-window-list)))
          (tmpbuf (get-buffer-create "*blacken*"))
          (errbuf (get-buffer-create "*blacken-error*")))
     ;; This buffer can be left after previous black invocation.  It
@@ -159,8 +161,9 @@ Show black output, if black exit abnormally and DISPLAY is t."
           (unless (eq (compare-buffer-substrings tmpbuf nil nil original-buffer nil nil) 0)
             (with-current-buffer tmpbuf
               (copy-to-buffer original-buffer (point-min) (point-max)))
-            (goto-char original-point)
-            (set-window-start (selected-window) original-window-pos))
+            (dolist (win-stt original-window-states)
+              (set-window-point (car win-stt) (nth 1 win-stt))
+              (set-window-start (car win-stt) (nth 2 win-stt))))
           (mapc 'kill-buffer (list tmpbuf errbuf)))
       (error (message "%s" (error-message-string err))
              (when display