In vc-print-branch-log use root instead of the default directory (bug#39704)

* lisp/vc/vc.el (vc-print-branch-log): Use rootdir instead of
default-directory for the second arg of vc-print-log-internal.

Author: link0ff

etc/NEWS

@@ -184,6 +184,9 @@ key             binding
 The original key binding was 'M-s', which interfered with I-search,
 since the latter uses 'M-s' as a prefix key of the search prefix map.
 
+** vc-print-branch-log shows the change log for BRANCH from its root
+directory instead of the default directory.
+
 
 * Incompatible Lisp Changes in Emacs 28.1
 

lisp/vc/vc.el

@@ -2558,15 +2558,17 @@ with its diffs (if the underlying VCS supports that)."
 
 ;;;###autoload
 (defun vc-print-branch-log (branch)
-  "Show the change log for BRANCH in a window."
+  "Show the change log for BRANCH root in a window."
   (interactive
    (list
     (vc-read-revision "Branch to log: ")))
   (when (equal branch "")
     (error "No branch specified"))
-  (vc-print-log-internal (vc-responsible-backend default-directory)
-                         (list default-directory) branch t
-                         (when (> vc-log-show-limit 0) vc-log-show-limit)))
+  (let* ((backend (vc-responsible-backend default-directory))
+         (rootdir (vc-call-backend backend 'root default-directory)))
+    (vc-print-log-internal backend
+                           (list rootdir) branch t
+                           (when (> vc-log-show-limit 0) vc-log-show-limit))))
 
 ;;;###autoload
 (defun vc-log-incoming (&optional remote-location)