emacs-31 afda8c2779b: Fix vc-hg-diff arguments to diff vs empty tree

branch: emacs-31
commit afda8c2779b297ada39379a38e0056cb2b21b1fb
Author: Aaron L. Zeng <[email protected]>
Commit: Sean Whitton <[email protected]>

    Fix vc-hg-diff arguments to diff vs empty tree
    
    * lisp/vc/vc-hg.el (vc-hg-diff): Fix behavior according to
    backend function specification (bug#81527).
    * test/lisp/vc/vc-hg-tests.el (vc-hg-diff-revision-arguments):
    New test.
---
 lisp/vc/vc-hg.el            |  2 +-
 test/lisp/vc/vc-hg-tests.el | 17 +++++++++++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/lisp/vc/vc-hg.el b/lisp/vc/vc-hg.el
index c0d92533241..51f353cdc29 100644
--- a/lisp/vc/vc-hg.el
+++ b/lisp/vc/vc-hg.el
@@ -567,7 +567,7 @@ This requires hg 4.4 or later, for the \"-L\" option of \"hg log\"."
     (when (and (not newvers) (member oldvers (list working ".")))
       (setq oldvers nil))
     (when (and newvers (not oldvers))
-      (setq oldvers working))
+      (setq oldvers "null"))
     (apply #'vc-hg-command
 	   (or buffer "*vc-diff*")
            (if async 'async 1)
diff --git a/test/lisp/vc/vc-hg-tests.el b/test/lisp/vc/vc-hg-tests.el
index 0216c23eebe..09891f10b22 100644
--- a/test/lisp/vc/vc-hg-tests.el
+++ b/test/lisp/vc/vc-hg-tests.el
@@ -88,4 +88,21 @@ R foo"
      ("foo2" added #s(vc-hg-extra-fileinfo renamed-from "foo"))
      ("bar2" added #s(vc-hg-extra-fileinfo renamed-from "bar")))))
 
+(ert-deftest vc-hg-diff-revision-arguments ()
+  "Test `vc-hg-diff' revision arguments."
+  (cl-letf (((symbol-function 'vc-hg-command) #'list)
+            (vc-hg-diff-switches t))
+    ;; REV1 REV2 both nil: diff file against working revision
+    (should (equal (vc-hg-diff '("foo") nil nil)
+                   '("*vc-diff*" 1 ("foo") "diff")))
+    ;; REV1 nil and REV2 non-nil: diff REV2 against empty tree
+    (should (equal (vc-hg-diff '("foo") nil "22222")
+                   '("*vc-diff*" 1 ("foo") "diff" "-r" "null" "-r" "22222")))
+    ;; REV1 non-nil and REV2 nil: diff working copy against older revision
+    (should (equal (vc-hg-diff '("foo") "11111" nil)
+                   '("*vc-diff*" 1 ("foo") "diff" "-r" "11111")))
+    ;; REV1 REV2 both non-nil: diff two revisions
+    (should (equal (vc-hg-diff '("foo") "11111" "22222")
+                   '("*vc-diff*" 1 ("foo") "diff" "-r" "11111" "-r" "22222")))))
+
 ;;; vc-hg-tests.el ends here