Fill the specified ref in webhook test payload (#20961) (#21055)

Backport #20961

The webhook payload should use the right ref when it‘s specified in the testing request.

The compare URL should not be empty, a URL like `compare/A...A` seems useless in most cases but is helpful when testing.

templates/swagger/v1_json.tmpl

@@ -4614,7 +4614,7 @@
           },
           {
             "type": "string",
-            "description": "The name of the commit/branch/tag. Default the repository’s default branch (usually master)",
+            "description": "The name of the commit/branch/tag, indicates which commit will be loaded to the webhook payload.",
             "name": "ref",
             "in": "query"
           }