New user option vc-dir-show-outgoing-count

* lisp/vc/vc-dir.el (vc-dir-show-outgoing-count): New defcustom.
(vc-dir-headers): Use it.
* doc/emacs/maintaining.texi (VC Directory Buffer):
* etc/NEWS: Document it.

doc/emacs/maintaining.texi

@@ -1528,6 +1528,17 @@ unpushed revisions"} where @var{N} is a number.  You can click on this
 text to execute the @code{vc-log-outgoing} command (@pxref{VC Change
 Log}).
 
+@vindex vc-dir-show-outgoing-count
+  Emacs tries to use cached information to determine the number of
+unpushed revisions, but for some backends this isn't possible.  In these
+cases Emacs must occasionally fetch from the remote repository in order
+to determine the count.  If your connection to the remote repository is
+slow then this may cause unacceptable slowdowns in refreshing the VC
+Directory buffer.  If this affects you, you can customize
+@code{vc-dir-show-outgoing-count} to nil to disable the unpushed
+revisions count altogether.  You can also set this on a per-repository
+basis using directory local variables (@pxref{Directory Variables}).
+
 @node VC Directory Commands
 @subsubsection VC Directory Commands
 

etc/NEWS

@@ -2551,6 +2551,7 @@ bindings:
 *** New display of outgoing revisions count in VC Directory.
 If there are outgoing revisions, VC Directory now includes a count of
 how many in its headers, to remind you to push them.
+You can disable this by customizing vc-dir-show-outgoing-count to nil.
 
 +++
 *** New user option 'vc-async-checkin' to enable async checkin operations.

lisp/vc/vc-dir.el

@@ -1339,6 +1339,17 @@ the *vc-dir* buffer.
   :doc "Local keymap for viewing outgoing revisions."
   "<down-mouse-1>" #'vc-log-outgoing)
 
+(defcustom vc-dir-show-outgoing-count t
+  "Whether to display the number of unpushed revisions in VC-Dir.
+For some combinations of VC backends and remotes, determining how many
+outgoing revisions there are is slow, because the backend must fetch
+from the remote, and your connection to the remote is slow.  Customize
+this variable to nil to disable calculating the outgoing count and
+therefore also disable the fetching."
+  :type 'boolean
+  :group 'vc
+  :version "31.1")
+
 (defun vc-dir-headers (backend dir)
   "Display the headers in the *VC-Dir* buffer.
 It calls the `dir-extra-headers' backend method to display backend
@@ -1351,7 +1362,8 @@ specific headers."
                'face 'vc-dir-header-value)
    (vc-call-backend backend 'dir-extra-headers dir)
    "\n"
-   (and-let* ((count (ignore-errors (vc--count-outgoing backend)))
+   (and-let* (vc-dir-show-outgoing-count
+              (count (ignore-errors (vc--count-outgoing backend)))
               (_ (plusp count)))
      (concat (propertize "Outgoing   : "
                          'face 'vc-dir-header)