New command vc-kill-other-working-tree-buffers

* lisp/vc/vc.el (vc-kill-other-working-tree-buffers): New
command.
* lisp/vc/vc-hooks.el (vc-prefix-map): Bind it.
* doc/emacs/vc1-xtra.texi (Other Working Trees):
* etc/NEWS: Document it.

lisp/vc/vc-hooks.el

@@ -968,6 +968,7 @@ In the latter case, VC mode is deactivated for this buffer."
   "!"   #'vc-edit-next-command
   "w c" #'vc-add-working-tree
   "w w" #'vc-switch-working-tree
+  "w k" #'vc-kill-other-working-tree-buffers
   "w s" #'vc-working-tree-switch-project
   "w x" #'vc-delete-working-tree
   "w R" #'vc-move-working-tree

lisp/vc/vc.el

@@ -4886,6 +4886,24 @@ MOVE non-nil means to move instead of copy."
     (message "Changes %s to `%s'"
              (if move "moved" "applied") directory)))
 
+;;;###autoload
+(defun vc-kill-other-working-tree-buffers (backend)
+  "Kill buffers visiting versions of this file in other working trees.
+BACKEND is the VC backend.
+
+This command kills the buffers that \\[vc-switch-working-tree] switches to,
+except that this command works only in file-visiting buffers."
+  (interactive (list (vc-responsible-backend default-directory)))
+  (when (cdr uniquify-managed)
+    (cl-loop with trees = (vc-call-backend backend
+                                           'known-other-working-trees)
+             for item in uniquify-managed
+             for buf = (uniquify-item-buffer item)
+             when (and (not (eq buf (current-buffer)))
+                       (cl-find (uniquify-item-dirname item) trees
+                                :test #'file-in-directory-p))
+             do (kill-buffer buf))))
+
 (defun vc-default-cherry-pick-comment (files rev reverse)
   (if reverse (format "Summary: Reverse-apply changes from revision %s\n\n"
                       rev)