merge: perform background file closing in batchget As 2fdbf22a1b63 demonstrated with stream clones, closing files on background threads on Windows can yield a significant speedup because closing files that have been created/appended to is slow on Windows/NTFS. Working directory updates can write thousands of files. Therefore it is susceptible to excessive slowness on Windows due to slow file closes. This patch enables background file closing when performing working directory file writes. The impact when performing an `hg up tip` on mozilla-central (136,357 files) from an empty working directory is significant: Before: 535s (8:55) After: 133s (2:13) Delta: -402s (6:42) That's a 4x speedup! By comparison, that same machine can perform the same operation in ~15s on Linux. So Windows went from ~35x to ~9x slower. Not bad but there's still work to do. As a reminder, background file closing is only activated on Windows because it is only beneficial on that platform. So this patch shouldn't change non-Windows behavior at all. It's worth noting that non-Windows systems perform working directory updates with multiple processes. Unfortunately, worker.py doesn't yet support Windows. So, there is still plenty of room for making working directory updates faster on Windows. Even if multiple processes are used on Windows, I believe background file closing will still provide a benefit, as individual processes will still be slowed down by the file close bottleneck (assuming the I/O system isn't saturated).