* lisp/vc/vc.el (vc-root-version-diff): New command (bug#33650).

(vc-root-diff): Call it interactively on a prefix argument.

doc/emacs/maintaining.texi

@@ -831,6 +831,14 @@ working tree containing the current VC fileset).  If you invoke this
 command from a Dired buffer, it applies to the working tree containing
 the directory.
 
+@findex vc-root-version-diff
+@kindex C-u C-x v D
+  To compare two arbitrary revisions of the whole trees, call
+@code{vc-root-diff} with a prefix argument: @kbd{C-u C-x v D}.  This
+prompts for two revision IDs (@pxref{VCS Concepts}), and displays a
+diff between those versions of the entire version-controlled directory
+trees (RCS, SCCS, CVS, and SRC do not support this feature).
+
 @vindex vc-diff-switches
   You can customize the @command{diff} options that @kbd{C-x v =} and
 @kbd{C-x v D} use for generating diffs.  The options used are taken
@@ -963,6 +971,7 @@ and the maximum number of revisions to display.
 Directory Mode}) or a Dired buffer (@pxref{Dired}), it applies to the
 file listed on the current line.
 
+@kindex C-x v L
 @findex vc-print-root-log
 @findex log-view-toggle-entry-display
   @kbd{C-x v L} (@code{vc-print-root-log}) displays a

etc/NEWS

@@ -398,6 +398,10 @@ with conflicts existed in earlier versions of Emacs, but incorrectly
 never detected a conflict due to invalid assumptions about cached
 values.
 
++++
+*** 'C-u C-x v D' ('vc-root-version-diff') prompts for two revisions
+and compares their entire trees.
+
 ** Diff mode
 *** Hunks are now automatically refined by default.
 To disable it, set the new defcustom 'diff-font-lock-refine' to nil.

lisp/vc/vc.el

@@ -1817,7 +1817,7 @@ Return t if the buffer had changes, nil otherwise."
 
 ;;;###autoload
 (defun vc-version-diff (_files rev1 rev2)
-  "Report diffs between revisions of the fileset in the repository history."
+  "Report diffs between REV1 and REV2 revisions of the fileset."
   (interactive (vc-diff-build-argument-list-internal))
   ;; All that was just so we could do argument completion!
   (when (and (not rev1) rev2)
@@ -1827,6 +1827,28 @@ Return t if the buffer had changes, nil otherwise."
   (vc-diff-internal t (vc-deduce-fileset t) rev1 rev2
 		    (called-interactively-p 'interactive)))
 
+;;;###autoload
+(defun vc-root-version-diff (_files rev1 rev2)
+  "Report diffs between REV1 and REV2 revisions of the whole tree."
+  (interactive (vc-diff-build-argument-list-internal))
+  ;; This is a mix of `vc-root-diff' and `vc-version-diff'
+  (when (and (not rev1) rev2)
+    (error "Not a valid revision range"))
+  (let ((backend (vc-deduce-backend))
+        (default-directory default-directory)
+        rootdir)
+    (if backend
+        (setq rootdir (vc-call-backend backend 'root default-directory))
+      (setq rootdir (read-directory-name "Directory for VC root-diff: "))
+      (setq backend (vc-responsible-backend rootdir))
+      (if backend
+          (setq default-directory rootdir)
+        (error "Directory is not version controlled")))
+    (let ((default-directory rootdir))
+      (vc-diff-internal
+       t (list backend (list rootdir)) rev1 rev2
+       (called-interactively-p 'interactive)))))
+
 ;;;###autoload
 (defun vc-diff (&optional historic not-urgent)
   "Display diffs between file revisions.
@@ -1900,10 +1922,8 @@ The optional argument NOT-URGENT non-nil means it is ok to say no to
 saving the buffer."
   (interactive (list current-prefix-arg t))
   (if historic
-      ;; FIXME: this does not work right, `vc-version-diff' ends up
-      ;; calling `vc-deduce-fileset' to find the files to diff, and
-      ;; that's not what we want here, we want the diff for the VC root dir.
-      (call-interactively 'vc-version-diff)
+      ;; We want the diff for the VC root dir.
+      (call-interactively 'vc-root-version-diff)
     (when buffer-file-name (vc-buffer-sync not-urgent))
     (let ((backend (vc-deduce-backend))
 	  (default-directory default-directory)