added a CLI command to obtain session ID

core/src/main/java/hudson/cli/SessionIdCommand.java

+package hudson.cli;
+
+import hudson.Extension;
+import jenkins.model.Jenkins;
+
+/**
+ * Prints the current session ID number (that changes for every run)
+ * to allow clients to reliably wait for a restart.
+ *
+ * @author Kohsuke Kawaguchi
+ */
+@Extension
+public class SessionIdCommand extends CLICommand {
+    @Override
+    public String getShortDescription() {
+        return "Outputs the session ID, which changes every time Jenkins restarts";
+    }
+
+    protected int run() {
+        stdout.println(Jenkins.SESSION_HASH);
+        return 0;
+    }
+}
+