Fix `magithub-fork-current' to not lose the original origin.

Instead, provide ways to either have it renamed to remote.upstream
automatically or edit the Git config file by hand. Or do just about
anything else.

magithub.el

@@ -1028,22 +1028,62 @@ printed as a message when the buffer is opened."
 
 ;;; Forking Repos
 
+(defvar magithub-fork-current-dwim nil
+  "When non-nil, silently modify user's config during in-place forking.
+When nil, the config file will be visited and you asked to edit
+it yourself inside a recursive edit level.
+
+You can toggle this variable temporarily by providing
+`magithub-fork-current' with an argument.
+
+You can also simply redefine `magithub-fork-current-callback' to
+completely change the behaviour and invalidate this doc string.
+
+By default, the original \"remote.origin\" section will be
+renamed to \"remote.upstream\" with its \"fetch\" config set to
+\"+refs/heads/*:refs/remotes/upstream/*\", and a new
+\"remote.origin\" section set up to point to the fork. Depending
+on your very own Git traits and customs, this might be great, or
+it might not (i.e., note that any additional settings you might
+have had in the original remote can become wrong after the
+rename).")
+
+(defun magithub-fork-current-callback (obj owner repo buffer dwim)
+  (with-current-buffer buffer
+    (magit-with-refresh
+      (let ((url (magithub-repo-url (car (magithub-auth-info)) repo 'ssh))
+            (fetch (format "+refs/heads/*:refs/remotes/%s/*"
+                           (if dwim "upstream" "origin"))))
+        (if dwim (progn (magit-git-string "config" "--rename-section"
+                                          "remote.origin" "remote.upstream")
+                        (magit-set fetch "remote" "upstream" "fetch")
+                        (magit-git-string "remote" "add" "origin" url))
+          (find-file
+           (expand-file-name "config"
+                             (magit-git-string "rev-parse" "--git-dir")))
+          (when (y-or-n-p "Make the automatic replacements? ")
+            (save-excursion (replace-regexp "\\(/\\|\"\\)origin\\1" "\\1upstream\\1"))
+            (undo-boundary)
+            (re-search-forward "^\\[remote \"upstream\"\\]" nil t)
+            (forward-line 0)
+            (insert "[remote \"origin\"]\n\turl = " url
+                    "\n\tfetch = " fetch ?\n))
+          (message "%s" (substitute-command-keys "Save the file and type\
+ \\[exit-recursive-edit] when finished"))
+          (recursive-edit)))))
+  (message "Forked %s/%s" owner repo))
+
 ;;;###autoload
-(defun magithub-fork-current ()
+(defun magithub-fork-current (&optional arg)
   "Fork the current repository in place."
-  (interactive)
+  (interactive "P")
   (destructuring-bind (owner repo _) (magithub-repo-info)
     (let ((url-request-method "POST"))
       (magithub-retrieve (list "repos" "fork" owner repo)
-                         (lambda (obj owner repo buffer)
-                           (with-current-buffer buffer
-                             (magit-with-refresh
-                               (magit-set (magithub-repo-url
-                                           (car (magithub-auth-info))
-                                           repo 'ssh)
-                                          "remote" "origin" "url")))
-                           (message "Forked %s/%s" owner repo))
-                         (list owner repo (current-buffer))))))
+                         #'magithub-fork-current-callback
+                         (list owner repo (current-buffer)
+                               (if arg (not magithub-fork-current-dwim)
+                                 magithub-fork-current-dwim))))))
 
 (defun magithub-send-pull-request (text recipients)
   "Send a pull request with text TEXT to RECIPIENTS.