fix off-by-one bug in FlatSet::difference_with

Summary:
The current implementation of `FlatSet::difference_with()` performs an
increment on the iterator returned by `m_set.erase()`. Since `m_set.erase()`
will move the iterator to the next element after the element being erased,
incrementing the iterator again creates an off-by-one bug, which manifests if
consecutive elements in `this` is also in `other`. Consequently we have the
following incorrect behavior:

```
sparta::FlatSet<int> s1{1, 2, 3, 4};
sparta::FlatSet<int> s2{1, 2, 3, 4};
std::cerr << s1.get_difference_with(s2) << std::endl; // expected {}, got {2, 4}
```

Reviewed By: anwesht, arnaudvenet

Differential Revision: D60269360

fbshipit-source-id: 7bb9a19eeab42e98bf2fd45b583ce5bcd21a8dd7

include/sparta/FlatSet.h

@@ -190,7 +190,6 @@ class FlatSet final
       if (it != end && Equal()(*it, *other_it)) {
         it = m_set.erase(it);
         end = m_set.end();
-        ++it;
       }
       ++other_it;
     }

test/SetTest.cpp

@@ -178,6 +178,11 @@ TYPED_TEST(UInt32SetTest, basicOperations) {
   TypeParam d43 = t4.get_difference_with(t3);
   EXPECT_THAT(d43,
               ::testing::UnorderedElementsAre(0, 1, 5, 101, 8137, 1234567));
+
+  std::vector<uint32_t> elements5 = {1, 2, 1023, 4096, 13001, bigint};
+  TypeParam t5(elements5.begin(), elements5.end());
+  TypeParam d54 = t5.get_difference_with(t4);
+  EXPECT_THAT(d54, ::testing::UnorderedElementsAre(1023, 13001));
 }
 
 TYPED_TEST(UInt32SetTest, robustness) {