t:listScmBrowsers was not working inside f:dropdownDescriptorSelector.

It assumed it was used only inside config-scm.jelly, so assumed it could use ${scmd}
rather than the more standard ${descriptor}.

core/src/main/resources/lib/hudson/listScmBrowsers.jelly

@@ -29,10 +29,11 @@ THE SOFTWARE.
       form field name.
     </st:attribute>
   </st:documentation>
+  <j:set var="browserDescriptors" value="${scmd != null ? scmd.browserDescriptors : descriptor.browserDescriptors}"/>
   <f:dropdownList name="${name}" title="${%Repository browser}" help="/help/scm-browsers/list.html">
     <f:dropdownListBlock value="auto" title="(${%Auto})" />
     <j:set var="currentBrowser" value="${scm.browser}"/>
-    <j:forEach var="descriptor" items="${scmd.browserDescriptors}" varStatus="loop">
+    <j:forEach var="descriptor" items="${browserDescriptors}" varStatus="loop">
       <j:set var="browser" value="${currentBrowser.descriptor==descriptor ? currentBrowser : null}"/><!-- for compatibility -->
       <f:dropdownListBlock value="${loop.index}" title="${descriptor.displayName}"
         selected="${currentBrowser.descriptor==descriptor}" staplerClass="${descriptor.clazz.name}"