lib/upgrade: Add html_url release field

lib/upgrade/upgrade_common.go

@@ -21,6 +21,10 @@ type Release struct {
 	Tag        string  `json:"tag_name"`
 	Prerelease bool    `json:"prerelease"`
 	Assets     []Asset `json:"assets"`
+
+	// The HTML URL is needed for human readable links in the output created
+	// by cmd/stupgrades.
+	HTMLURL string `json:"html_url"`
 }
 
 type Asset struct {