commit 71fd3f800f88676ff3393852464197a86a28ed22
Author: Tom Tromey <tom@tromey.com>
Date:   Mon Feb 13 17:24:04 2017 -0700

    Make vc-git and vc-hg revert buffers on branch switch
    
    * lisp/vc/vc-git.el (vc-git-retrieve-tag): Respect "update".
    * lisp/vc/vc-hg.el (vc-hg-retrieve-tag): Respect "update".

diff --git a/lisp/vc/vc-git.el b/lisp/vc/vc-git.el
index de07ea2..586394b 100644
--- a/lisp/vc/vc-git.el
+++ b/lisp/vc/vc-git.el
@@ -1238,11 +1238,14 @@ vc-git-create-tag
              (vc-git-command nil 0 nil "checkout" "-b" name)
            (vc-git-command nil 0 nil "tag" name)))))
 
-(defun vc-git-retrieve-tag (dir name _update)
+(declare-function vc-resynch-buffers-in-directory "vc-dispatcher"
+                  (directory &optional keep noquery reset-vc-info))
+
+(defun vc-git-retrieve-tag (dir name update)
   (let ((default-directory dir))
     (vc-git-command nil 0 nil "checkout" name)
-    ;; FIXME: update buffers if `update' is true
-    ))
+    (when update
+      (vc-resynch-buffers-in-directory dir t t))))
 
 
 ;;; MISCELLANEOUS
diff --git a/lisp/vc/vc-hg.el b/lisp/vc/vc-hg.el
index 2f9487c..ea61494 100644
--- a/lisp/vc/vc-hg.el
+++ b/lisp/vc/vc-hg.el
@@ -558,11 +558,15 @@ vc-hg-create-tag
     (and (vc-hg-command nil 0 nil "status")
          (vc-hg-command nil 0 nil (if branchp "bookmark" "tag") name))))
 
-(defun vc-hg-retrieve-tag (dir name _update)
+(declare-function vc-resynch-buffers-in-directory "vc-dispatcher"
+                  (directory &optional keep noquery reset-vc-info))
+
+(defun vc-hg-retrieve-tag (dir name update)
   "Retrieve the version tagged by NAME of all registered files at or below DIR."
   (let ((default-directory dir))
     (vc-hg-command nil 0 nil "update" name)
-    ;; FIXME: update buffers if `update' is true
+    (when update
+      (vc-resynch-buffers-in-directory dir t t))
     ;; TODO: update *vc-change-log* buffer so can see @ if --graph
     ))