tests: stabilize test-persistent-nodemap.t on Windows Several issues here: - Hooks can't invoke shell scripts on Windows, so use `sh` to launch - `dd` in MSYS only recognizes `status=noxfer` - The `PATH` updating triggered a massive slowdown, but is no longer needed I have no idea why, but removing the `PATH` update substantially increased the speed of the test. It was running finishing at ~4:30 with `--debug` and ~14:50 without it, but now completes in ~2:20 on my Windows laptop. Differential Revision: https://phab.mercurial-scm.org/D10636

local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed
findsnapshot test passed