try alternate GetExistingRules() query

per https://stackoverflow.com/a/51931004/2343739

ref #357

src/FilterLists.Services/Snapshot/SnapshotBatch.cs

@@ -1,5 +1,4 @@
-using System;
-using System.Collections.Generic;
+using System.Collections.Generic;
 using System.Linq;
 using System.Threading.Tasks;
 using FilterLists.Data;
@@ -33,23 +32,13 @@ public async Task SaveAsync()
             await dbContext.SaveChangesAsync();
         }
 
-        private IQueryable<Rule> GetExistingRules()
-        {
-            var lineRules = lines.Select(l => new Rule {Raw = l});
-            return dbContext.Rules.Intersect(lineRules, new RuleRawEqualityComparer());
-        }
+        private IQueryable<Rule> GetExistingRules() =>
+            dbContext.Rules.Join(lines, rule => rule.Raw, line => line, (rule, line) => rule);
 
         private List<Rule> CreateNewRules(IQueryable<Rule> existingRules) =>
             lines.Except(existingRules.Select(r => r.Raw)).Select(r => new Rule {Raw = r}).ToList();
 
         private void AddSnapshotRules(IQueryable<Rule> rules) =>
             snapEntity.AddedSnapshotRules.AddRange(rules.Select(r => new SnapshotRule {Rule = r}));
-
-        private class RuleRawEqualityComparer : IEqualityComparer<Rule>
-        {
-            public bool Equals(Rule x, Rule y) => x?.Raw == y?.Raw;
-
-            public int GetHashCode(Rule obj) => throw new NotImplementedException();
-        }
     }
 }