Have 'vc-prepare-patch' handle prefix arguments.

* lisp/emacs-lisp/package-vc.el (package-vc-prepare-patch): Use
'vc-prepare-patch-prompt-revisions'.
* lisp/vc/vc.el (vc-prepare-patch-prompt-revisions): Extract common
function and handle prefix arguments.
(vc-prepare-patch): Pull logic out to
'vc-prepare-patch-prompt-revisions'.

lisp/emacs-lisp/package-vc.el

@@ -750,20 +750,20 @@ prompt for the name of the package to rebuild."
   (package-vc--unpack-1 pkg-desc (package-desc-dir pkg-desc)))
 
 ;;;###autoload
-(defun package-vc-prepare-patch (pkg subject revisions)
+(defun package-vc-prepare-patch (pkg-desc subject revisions)
   "Send patch for REVISIONS to maintainer of the package PKG using SUBJECT.
-SUBJECT and REVISIONS are passed on to `vc-prepare-patch', which see.
+The function uses `vc-prepare-patch', passing SUBJECT and
-PKG must be a package description.
+REVISIONS directly.  PKG-DESC must be a package description.
-Interactively, prompt for PKG, SUBJECT, and REVISIONS.  However,
+Interactively, prompt for PKG-DESC, SUBJECT, and REVISIONS.  When
-if the current buffer has marked commit log entries, REVISIONS
+invoked with a numerical prefix argument, use the last N
-are the tags of the marked entries, see `log-view-get-marked'."
+revisions.  When invoked interactively in a Log View buffer with
+marked revisions, use those."
   (interactive
    (list (package-vc--read-package-desc "Package to prepare a patch for: " t)
          (and (not vc-prepare-patches-separately)
               (read-string "Subject: " "[PATCH] " nil nil t))
-         (or (log-view-get-marked)
+         (vc-prepare-patch-prompt-revisions)))
-             (vc-read-multiple-revisions "Revisions: "))))
+  (vc-prepare-patch (package-maintainers pkg-desc t)
-  (vc-prepare-patch (package-maintainers pkg t)
                     subject revisions))
 
 (provide 'package-vc)

lisp/vc/vc.el

@@ -3384,25 +3384,43 @@ If nil, no default will be used.  This option may be set locally."
                                (vc-root-dir))))
             :buffer (current-buffer)))))
 
+(defun vc-prepare-patch-prompt-revisions ()
+  "Prompt the user for a list revisions.
+Prepare a default value, depending on the current context.  With
+a numerical prefix argument, use the last N revisions as the
+default value.  If the current buffer is a log-view buffer, use
+the marked commits.  Otherwise fall back to the working revision
+of the current file."
+  (vc-read-multiple-revisions
+   "Revisions: " nil nil nil
+   (or (and-let* ((arg current-prefix-arg)
+                  (fs (vc-deduce-fileset t)))
+         (cl-loop with file = (caadr fs)
+                  repeat (prefix-numeric-value arg)
+                  for rev = (vc-working-revision file)
+                  then (vc-call-backend
+                        (car fs) 'previous-revision
+                        file rev)
+                  when rev collect it into revs
+                  finally return (mapconcat #'identity revs ",")))
+       (and-let* ((revs (log-view-get-marked)))
+         (mapconcat #'identity revs ","))
+       (and-let* ((file (buffer-file-name)))
+         (vc-working-revision file)))))
+
 ;;;###autoload
 (defun vc-prepare-patch (addressee subject revisions)
   "Compose an Email sending patches for REVISIONS to ADDRESSEE.
-If `vc-prepare-patches-separately' is nil, SUBJECT will be used
+If `vc-prepare-patches-separately' is nil, use SUBJECT as the
-as the default subject for the message (and it will be prompted
+default subject for the message, or prompt a subject when invoked
-for when called interactively).  Otherwise a separate message
+interactively.  Otherwise compose a separate message for each
-will be composed for each revision, with SUBJECT derived from the
+revision, with SUBJECT derived from each revision subject.
-invidividual commits.
+When invoked with a numerical prefix argument, use the last N
-
+revisions.
-When invoked interactively in a Log View buffer with marked
+When invoked interactively in a Log View buffer with
-revisions, those revisions will be used."
+marked revisions, use those these."
   (interactive
-   (let ((revs (vc-read-multiple-revisions
+   (let ((revs (vc-prepare-patch-prompt-revisions)) to)
-                "Revisions: " nil nil nil
-                (or (and-let* ((revs (log-view-get-marked)))
-                      (mapconcat #'identity revs ","))
-                    (and-let* ((file (buffer-file-name)))
-                      (vc-working-revision file)))))
-         to)
      (require 'message)
      (while (null (setq to (completing-read-multiple
                             (format-prompt