vc-next-action: Leave files unregistered if user aborts the checkin

* lisp/vc/vc.el (vc-only-files-state-and-model):
Don't call vc-register.
(vc-checkin): New REGISTER parameter.
Call vc-register when it's non-nil.
(vc-next-action): Pass new REGISTER parameter to vc-checkin.

etc/NEWS

@@ -1777,10 +1777,10 @@ These correspond to the existing 'z p' to pop a stash and 'P' to pop the
 stash at point (deleting the stash at point is also bound to 'C-k').
 
 ---
-*** VC Directory now offers to register files when checking in mixed filesets.
+*** VC Directory can now register files when checking in mixed filesets.
 Previously, if some files to be checked in were unregistered but others
 were added, removed or edited, Emacs would refuse to proceed.
-Now Emacs prompts to first register the unregistered files, so that all
+Now Emacs prompts to register the unregistered files, so that all
 files in the fileset are in a compatible state for a checkin.
 
 +++

lisp/vc/vc.el

@@ -1377,12 +1377,25 @@ BACKEND is the VC backend responsible for FILES."
           ((cl-subsetp states '(added missing removed edited))
            (setq state 'edited))
 
-          ;; Special, but common case:
-          ;; checking in both changes and new files at once.
-          ((and (cl-subsetp states '(added missing removed edited unregistered))
-                (y-or-n-p "Some files are unregistered; register them first?"))
-           (vc-register (list backend
-                              (cdr (assq 'unregistered states-alist))))
+          ;; Special, but common case: checking in both changes and new
+          ;; files at once.  The actual registration is delayed until
+          ;; `vc-checkin' so that if the user changes their mind while
+          ;; entering the log message, we leave things as we found them.
+          ;;
+          ;; An alternative would be to delay it until the backend
+          ;; `vc-*-checkin'.  The advantages would be that those
+          ;; functions could complete the whole operation in fewer total
+          ;; shell commands, and if the checkin itself fails they could
+          ;; ensure the file is left unregistered then too (e.g. for Git
+          ;; we may be able to use 'git add -N', though that would still
+          ;; require a subsequent 'git reset').
+          ;; The disadvantage would be a more complex VC API because we
+          ;; would have to distinguish between backends which can and
+          ;; can't handle registration and checkin together.
+          ((and (cl-subsetp states
+                            '(added missing removed edited unregistered))
+                (y-or-n-p "\
+Some files are unregistered; register them before checking in?"))
            (setq state 'edited))
 
           (t
@@ -1470,7 +1483,7 @@ from which to check out the file(s)."
   (let* ((vc-fileset (vc-deduce-fileset nil t 'state-model-only-files))
          (backend (car vc-fileset))
 	 (files (nth 1 vc-fileset))
-         ;; (fileset-only-files (nth 2 vc-fileset))
+         (fileset-only-files (nth 2 vc-fileset))
          ;; FIXME: We used to call `vc-recompute-state' here.
          (state (nth 3 vc-fileset))
          ;; The backend should check that the checkout-model is consistent
@@ -1550,6 +1563,9 @@ from which to check out the file(s)."
 		  (with-current-buffer visited
 		    (read-only-mode -1)))))))
 	;; Allow user to revert files with no changes
+        ;; FIXME: This will never do anything because STATE will never
+        ;; be `up-to-date' in this branch of the cond.
+        ;; How did the code end up like this?  --spwhitton
 	(save-excursion
           (let (to-revert)
             (dolist (file files)
@@ -1570,17 +1586,22 @@ from which to check out the file(s)."
 	;; Remaining files need to be committed
 	(if (not ready-for-commit)
 	    (message "No files remain to be committed")
-	  (if (not verbose)
-	      (vc-checkin ready-for-commit backend)
-	    (let* ((revision (read-string "New revision or backend: "))
-                   (revision-downcase (downcase revision)))
-	      (if (member
-		   revision-downcase
-		   (mapcar (lambda (arg) (downcase (symbol-name arg)))
-			   vc-handled-backends))
-		  (let ((vsym (intern revision-downcase)))
-		    (dolist (file files) (vc-transfer-file file vsym)))
-		(vc-checkin ready-for-commit backend nil nil revision)))))))
+          ;; In the case there actually are any unregistered files then
+          ;; `vc-deduce-backend', via `vc-only-files-state-and-model',
+          ;; has already prompted the user to approve registering them.
+	  (let ((register (cl-remove-if #'vc-backend fileset-only-files)))
+            (if (not verbose)
+	        (vc-checkin ready-for-commit backend nil nil nil nil register)
+	      (let* ((revision (read-string "New revision or backend: "))
+                     (revision-downcase (downcase revision)))
+	        (if (member
+		     revision-downcase
+		     (mapcar (lambda (arg) (downcase (symbol-name arg)))
+			     vc-handled-backends))
+		    (let ((vsym (intern revision-downcase)))
+		      (dolist (file files) (vc-transfer-file file vsym)))
+		  (vc-checkin ready-for-commit backend
+                              nil nil revision nil register))))))))
      ;; locked by somebody else (locking VCSes only)
      ((stringp state)
       ;; In the old days, we computed the revision once and used it on
@@ -1891,7 +1912,8 @@ Type \\[vc-next-action] to check in changes.")
      (substitute-command-keys
       "Please explain why you stole the lock.  Type \\`C-c C-c' when done"))))
 
-(defun vc-checkin (files backend &optional comment initial-contents rev patch-string)
+(defun vc-checkin
+    (files backend &optional comment initial-contents rev patch-string register)
   "Check in FILES.
 
 COMMENT is a comment string; if omitted, a buffer is popped up to accept
@@ -1901,6 +1923,9 @@ initial contents of the log entry buffer.
 The optional argument REV may be a string specifying the new revision
 level (only supported for some older VCSes, like RCS and CVS).
 The optional argument PATCH-STRING is a string to check in as a patch.
+If the optional argument REGISTER is non-nil, it should be a list of
+files to register before checking in; if any of these are already
+registered the checkin will abort.
 
 Runs the normal hooks `vc-before-checkin-hook' and `vc-checkin-hook'."
   (run-hooks 'vc-before-checkin-hook)
@@ -1917,6 +1942,7 @@ Runs the normal hooks `vc-before-checkin-hook' and `vc-checkin-hook'."
       ;; RCS 5.7 gripes about whitespace-only comments too.
       (unless (and comment (string-match "[^\t\n ]" comment))
         (setq comment "*** empty log message ***"))
+      (when register (vc-register (list backend files)))
       (cl-labels ((do-it ()
                     ;; We used to change buffers to get local value of
                     ;; `vc-checkin-switches', but the (singular) local