Look at notes created just before merge when deciding if an MR can be reverted

On MySQL, at least, `Note#created_at` doesn't seem to store fractional seconds,
while `MergeRequest::Metrics#merged_at` does. This breaks the optimization
assumption that we only need to search for notes created *after* the MR has
been merged.

Unsynchronized system clocks also make this a dangerous assumption to make.

Adding a minute of leeway still optimizes away most notes, but allows both
cases to be handled more gracefully. If the system clocks are more than a
minute out, we'll still be broken, of course.

app/models/merge_request.rb

@@ -989,8 +989,14 @@ class MergeRequest < ActiveRecord::Base
     merged_at = metrics&.merged_at
     notes_association = notes_with_associations
+    # It is not guaranteed that Note#created_at will be strictly later than
+    # MergeRequestMetric#merged_at. Nanoseconds on MySQL may break this
+    # comparison, as will a HA environment if clocks are not *precisely*
+    # synchronized. Add a minute's leeway to compensate for both possibilities
+    cutoff = merged_at - 1.minute
+
     if merged_at
-      notes_association = notes_association.where('created_at > ?', merged_at)
+      notes_association = notes_association.where('created_at >= ?', cutoff)
     end
     !merge_commit.has_been_reverted?(current_user, notes_association)

spec/models/merge_request_spec.rb

@@ -1127,9 +1127,19 @@ describe MergeRequest do
           end
         end
-        context 'when the revert commit is mentioned in a note before the MR was merged' do
+        context 'when the revert commit is mentioned in a note just before the MR was merged' do
           before do
-            subject.notes.last.update!(created_at: subject.metrics.merged_at - 1.second)
+            subject.notes.last.update!(created_at: subject.metrics.merged_at - 30.seconds)
+          end
+
+          it 'returns false' do
+            expect(subject.can_be_reverted?(current_user)).to be_falsey
+          end
+        end
+
+        context 'when the revert commit is mentioned in a note long before the MR was merged' do
+          before do
+            subject.notes.last.update!(created_at: subject.metrics.merged_at - 2.minutes)
           end
           it 'returns true' do