template: factor onTemplateRendered out of template re-renderer

Pull this function out so it can be used by the first-pass renderer when
we restore a task from a restarted client.

client/allocrunner/taskrunner/template/template.go

@@ -368,6 +368,12 @@ func (tm *TaskTemplateManager) handleTemplateRerenders(allRenderedTime time.Time
 					SetFailsTask().
 					SetDisplayMessage(fmt.Sprintf("Template failed: %v", err)))
 		case <-tm.runner.TemplateRenderedCh():
+			tm.onTemplateRendered(handledRenders, allRenderedTime)
+		}
+	}
+}
+
+func (tm *TaskTemplateManager) onTemplateRendered(handledRenders map[string]time.Time, allRenderedTime time.Time) {
 	// A template has been rendered, figure out what to do
 	var handling []string
 	signals := make(map[string]struct{})
@@ -472,8 +478,7 @@ func (tm *TaskTemplateManager) handleTemplateRerenders(allRenderedTime time.Time
 			}
 		}
 	}
-		}
-	}
+
 }
 
 // allTemplatesNoop returns whether all the managed templates have change mode noop.