Improve Date Filter performance

[rubberduck203]

Currently, we perform filtering by walking every commit in history, then filtering the commits after the fact.

GitNStats/src/gitnstats/Controller.cs

Lines 56 to 57 in 116f2b9

	var diffs = await AsyncVisitorFactory(repo).Walk(branch.Tip);
	var filteredDiffs = diffs.Where(diffFilter.Invoke);

It would be far more efficient, particularly when analyzing repos with long histories, if we had a different visitor that stopped walking the history once it hit the date we're looking for.