[FIXED JENKINS-22934] Show poll thread count option when necessary.

* Show the option when the configured value is not the default
* Show the option even when projects are in other ItemGroups
* Only consider AbstractProjects

core/src/main/java/hudson/triggers/SCMTrigger.java

@@ -45,6 +45,8 @@
import hudson.util.SequentialExecutionQueue;
import org.apache.commons.io.FileUtils;
import org.apache.commons.jelly.XMLOutput;
import org.kohsuke.accmod.Restricted;
import org.kohsuke.accmod.restrictions.NoExternalUse;
import org.kohsuke.stapler.StaplerRequest;
import org.kohsuke.stapler.DataBoundConstructor;

@@ -260,6 +262,15 @@ public void setPollingThreadCount(int n) {
            resizeThreadPool();
        }

        @Restricted(NoExternalUse.class)
        public boolean isPollingThreadCountOptionVisible() {
            // unless you have a fair number of projects, this option is likely pointless.
            // so let's hide this option for new users to avoid confusing them
            // unless it was already changed
            return Jenkins.getInstance().getAllItems(AbstractProject.class).size() > 10
                    || getPollingThreadCount() != 0;
        }

        /**
         * Update the {@link ExecutorService} instance.
         */

core/src/main/resources/hudson/triggers/SCMTrigger/global.jelly

@@ -24,11 +24,7 @@ THE SOFTWARE.

<?jelly escape-by-default='true'?>
<j:jelly xmlns:j="jelly:core" xmlns:st="jelly:stapler" xmlns:d="jelly:define" xmlns:l="/lib/layout" xmlns:t="/lib/hudson" xmlns:f="/lib/form">
  <j:if test="${app.items.size()>10}">
    <!--
      unless you have a fair number of projects, this option is likely pointless.
      so let's hide this option for new users to avoid confusing them.
    -->
  <j:if test="${descriptor.pollingThreadCountOptionVisible}">
    <f:section title="${%SCM Polling}">
      <f:entry title="${%Max # of concurrent polling}" field="pollingThreadCount">
        <f:number value="${descriptor.pollingThreadCount==0 ? '' : descriptor.pollingThreadCount}"