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.