[JENKINS-35906] Ensure that SCMDescriptor.newInstance overrides are honored (#2426)

* [FIXED JENKINS-35906] Ensure that SCMDescriptor.newInstance overrides are honored.

* [FIXED JENKINS-36043] Work around fragile form submission design in multi-branch-project-plugin.

(cherry picked from commit 1d176d1f)

core/src/main/java/hudson/scm/SCMS.java

@@ -56,7 +56,10 @@ public class SCMS {
      */
     @SuppressWarnings("deprecation")
     public static SCM parseSCM(StaplerRequest req, AbstractProject target) throws FormException, ServletException {
-        SCM scm = req.bindJSON(SCM.class, req.getSubmittedForm().getJSONObject("scm"));
+        SCM scm = SCM.all().newInstanceFromRadioList(req.getSubmittedForm().getJSONObject("scm"));
+        if (scm == null) {
+            scm = new NullSCM(); // JENKINS-36043 workaround for AbstractMultiBranchProject.submit
+        }
         scm.getDescriptor().generation++;
         return scm;
     }