worker: partition a list (of tasks) into equal-sized chunks

worker: estimate whether it's worth running a task in parallel Not implemented for Windows yet.

worker: count the number of CPUs This works on the major platforms, and falls back to a safe guess of 1 elsewhere.

tests: getremove test output changes (fold into previous patch)

merge: report non-interactive progress in chunks Instead of a monotonic count, getupdates yields the number of files it has updated since it last reported, and its caller sums the numbers when updating progress. Once we run these updates in parallel, this will allow worker processes to report progress less often, reducing overhead.

merge: handle subrepo merges and .hgsubstate specially In an upcoming patch, we will update .hgsubstate in a non-interactive worker process. Merges of subrepo contents will still need to occur in the master process (since they may be interactive), so we move that code into a place where it will always run in what will become the master process.

tests: update test output (will be folded into parent)

merge: split out mostly-non-interactive working dir updates In a later patch, we'll run these updates in parallel.