Use unsafe merge in joinResults.addChildAndChildren()

[bboreham]

This was missed from #3138. It's a bug as-is - Merge() can return the rhs which is not safe to do further 'unsafe' operations on.

Benchmarks show the impact is mixed:

benchmark                         old ns/op     new ns/op     delta
BenchmarkRenderList-2             553973514     505759984     -8.70%
BenchmarkRenderHosts-2            345194045     366304720     +6.12%
BenchmarkRenderControllers-2      314828412     309772476     -1.61%
BenchmarkRenderPods-2             283650521     313774503     +10.62%
BenchmarkRenderContainers-2       187017594     194096252     +3.79%
BenchmarkRenderProcesses-2        121727679     111295296     -8.57%
BenchmarkRenderProcessNames-2     127764403     121302933     -5.06%

benchmark                         old allocs     new allocs     delta
BenchmarkRenderList-2             684645         621168         -9.27%
BenchmarkRenderHosts-2            486703         497536         +2.23%
BenchmarkRenderControllers-2      376311         358656         -4.69%
BenchmarkRenderPods-2             325505         334824         +2.86%
BenchmarkRenderContainers-2       221536         221534         -0.00%
BenchmarkRenderProcesses-2        104514         104513         -0.00%
BenchmarkRenderProcessNames-2     142520         130292         -8.58%

benchmark                         old bytes     new bytes     delta
BenchmarkRenderList-2             76111276      69272488      -8.99%
BenchmarkRenderHosts-2            50265090      51547970      +2.55%
BenchmarkRenderControllers-2      39539499      37658241      -4.76%
BenchmarkRenderPods-2             33302721      34403579      +3.31%
BenchmarkRenderContainers-2       23492377      23494531      +0.01%
BenchmarkRenderProcesses-2        11504046      11503747      -0.00%
BenchmarkRenderProcessNames-2     14845492      13599020      -8.40%

Also comment the add() call introduced in #3135 is inconsistent with #3138 - no impact as the two sets of calls are never used together, but it's not ideal. Changing add() to copy children makes performance worse.