"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.
