merge: increase safety of parallel updating/removing on icasefs "merge.applyupdates()" sorts "actions" in removal first order, and "workeractions" derived from it should be also sorted. If each actions in "workeractions" are executed in serial, this sorting ensures that merging/updating process is collision free, because updating the file in target context is always executed after removing the existing file which causes case-folding collision against the former. In the other hand, if each actions are executed in parallel, updating on a worker process may be executed before removing on another worker process, because "worker.partition()" partitions list of actions regardless of type of each actions. This patch divides "workeractions" into removing and updating, and executes the former first. This patch still scans "actions"/"workeractions" some times for ease of patch review, even though large list may cost much in this way. (total cost should be as same as before) This also changes some tests, because dividing "workeractions" affects progress indication.