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.
pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2
pager.pager=p2 # $PAGER
ui.editor=e1 # $TESTTMP/userrc:2