fix group by date issue #7

Sort commits before grouping by date. Only incur the sort cost
for sorting by date.

Walrus.Core/WalrusService.cs

@@ -50,6 +50,7 @@ public IEnumerable<CommitGroup> ExecuteQuery(WalrusQuery query)
                         g.OrderBy(c => c.Timestamp))),
 
                 WalrusQuery.QueryGrouping.Date => commits
+                    .OrderBy(c => c.Timestamp)
                     .GroupBy(c => c.Timestamp.Date)
                     .Select(g => new CommitGroup(g.Key, 
                         g.OrderBy(c => c.Timestamp))),