VC outgoing commands for Git: Don't unconditionally fetch

* lisp/vc/vc-bzr.el (vc-bzr-incoming-revision):
* lisp/vc/vc-hg.el (vc-hg-incoming-revision):
* lisp/vc/vc.el (vc-diff-incoming, vc--incoming-revision): New
REFRESH optional argument.
(vc-default-log-incoming): Pass it.
* lisp/vc/vc-git.el (vc-git-incoming-revision): New REFRESH
optional argument.  When nil, use cached info (bug#62940).

lisp/vc/vc-bzr.el

@@ -822,7 +822,7 @@ If LIMIT is non-nil, show no more than this many entries."
 	 (list "--theirs-only" (and (not (string-empty-p remote-location))
                                     remote-location))))
 
-(defun vc-bzr-incoming-revision (remote-location)
+(defun vc-bzr-incoming-revision (remote-location &optional _refresh)
   (with-temp-buffer
     (vc-bzr-command "missing" t 1 nil
                     "--log-format=long" "--show-ids"

lisp/vc/vc-git.el

@@ -70,7 +70,7 @@
 ;; - get-change-comment (files rev)                OK
 ;; HISTORY FUNCTIONS
 ;; * print-log (files buffer &optional shortlog start-revision limit)   OK
-;; * incoming-revision (remote-location)           OK
+;; * incoming-revision (remote-location &optional refresh)   OK
 ;; - log-search (buffer pattern)                   OK
 ;; - log-view-mode ()                              OK
 ;; - show-log-entry (revision)                     OK
@@ -1605,19 +1605,20 @@ If LIMIT is a non-empty string, use it as a base revision."
                               start-revision))
 		'("--")))))))
 
-(defun vc-git-incoming-revision (remote-location)
-  (vc-git-command nil 0 nil "fetch"
-                  (and (not (string-empty-p remote-location))
-                       ;; Extract remote from "remote/branch".
-                       (replace-regexp-in-string "/.*" ""
-                                                 remote-location)))
-  (ignore-errors              ; in order to return nil if no such branch
-    (with-output-to-string
-      (vc-git-command standard-output 0 nil
-                      "log" "--max-count=1" "--pretty=format:%H"
-                      (if (string-empty-p remote-location)
-			  "@{upstream}"
-		        remote-location)))))
+(defun vc-git-incoming-revision (remote-location &optional refresh)
+  (let ((rev (if (string-empty-p remote-location)
+		 "@{upstream}"
+	       remote-location)))
+    (when (or refresh (null (vc-git--rev-parse rev)))
+      (vc-git-command nil 0 nil "fetch"
+                      (and (not (string-empty-p remote-location))
+                           ;; Extract remote from "remote/branch".
+                           (replace-regexp-in-string "/.*" ""
+                                                     remote-location))))
+    (ignore-errors            ; in order to return nil if no such branch
+      (with-output-to-string
+        (vc-git-command standard-output 0 nil
+                        "log" "--max-count=1" "--pretty=format:%H" rev)))))
 
 (defun vc-git-log-search (buffer pattern)
   "Search the log of changes for PATTERN and output results into BUFFER.

lisp/vc/vc-hg.el

@@ -1531,7 +1531,7 @@ This runs the command \"hg summary\"."
          (nreverse result))
        "\n"))))
 
-(defun vc-hg-incoming-revision (remote-location)
+(defun vc-hg-incoming-revision (remote-location &optional _refresh)
   (let* ((remote-location (if (string-empty-p remote-location)
                               "default"
                             remote-location))

lisp/vc/vc.el

@@ -408,13 +408,16 @@
 ;;   received when performing a pull operation from REMOTE-LOCATION.
 ;;   Deprecated: implement incoming-revision and mergebase instead.
 ;;
-;; * incoming-revision (remote-location)
+;; * incoming-revision (remote-location &optional refresh)
 ;;
 ;;   Return revision at the head of the branch at REMOTE-LOCATION.
 ;;   If there is no such branch there, return nil.  (Should signal an
 ;;   error, not return nil, in the case that fetching data fails.)
 ;;   For a distributed VCS, should also fetch that revision into local
 ;;   storage for operating on by subsequent calls into the backend.
+;;   The backend may rely on cached information from a previous fetch
+;;   from REMOTE-LOCATION unless REFRESH is non-nil, which means that
+;;   the most up-to-date information possible is required.
 ;;
 ;; - log-search (buffer pattern)
 ;;
@@ -2562,7 +2565,8 @@ global binding."
   (let* ((fileset (or fileset (vc-deduce-fileset t)))
          (backend (car fileset))
          (incoming (vc--incoming-revision backend
-                                          (or remote-location ""))))
+                                          (or remote-location "")
+                                          'refresh)))
     (vc-diff-internal vc-allow-async-diff fileset
                       (vc-call-backend backend 'mergebase incoming)
                       incoming
@@ -3548,8 +3552,8 @@ The command prompts for the branch whose change log to show."
        (read-string "Remote location/branch (empty for default): " nil
                     'vc-remote-location-history)))
 
-(defun vc--incoming-revision (backend remote-location)
-  (or (vc-call-backend backend 'incoming-revision remote-location)
+(defun vc--incoming-revision (backend remote-location &optional refresh)
+  (or (vc-call-backend backend 'incoming-revision remote-location refresh)
       (user-error "No incoming revision -- local-only branch?")))
 
 ;;;###autoload
@@ -3565,7 +3569,7 @@ In some version control systems REMOTE-LOCATION can be a remote branch name."
 
 (defun vc-default-log-incoming (_backend buffer remote-location)
   (vc--with-backend-in-rootdir ""
-    (let ((incoming (vc--incoming-revision backend remote-location)))
+    (let ((incoming (vc--incoming-revision backend remote-location 'refresh)))
       (vc-call-backend backend 'print-log (list rootdir) buffer t
                        incoming
                        (vc-call-backend backend 'mergebase incoming)))))