sparse: move working directory refreshing into core
This is a pretty straightforward move of the code.
I converted the "force" argument to a keyword argument.
Like other recent changes, this code is tightly coupled with
working directory update code in merge.py. I suspect the code
will become more tightly coupled over time, possibly even moved
to merge.py. For now, let's get the code in core.
warning
error
'buffered\n'
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