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.

sparse: refactor update actions filtering and call from core merge.calculateupdates() now filters the update actions through sparse by default. The filtering no-ops if sparse isn't enabled or no sparse config is defined. The function has been refactored to behave more like a filter instead of a wrapper of merge.calculateupdates(). We should arguably take sparse into account earlier in merge.calculateupdates(). This patch preserves the old behavior of applying sparse at the end of update calculation, which is the simplest and safest approach.

sparse: move update action filtering into core This is a relatively straight port of the function. It is pretty large. So refactoring will be postponed to a subsequent commit.