Use the same way to invalidate cache in EE

Because it doesn't really hurt and could reduce conflicts
Discussion:
https://gitlab.com/gitlab-org/gitlab-ee/commit/5020e1a779cecbf16fffa3b37726b9167093126c#note_32403646

app/services/issuable_base_service.rb

@@ -236,8 +236,9 @@ class IssuableBaseService < BaseService
         )
 
         if old_assignees != issuable.assignees
-          assignees = old_assignees + issuable.assignees.to_a
-          invalidate_cache_counts(assignees.compact, issuable)
+          new_assignees = issuable.assignees.to_a
+          affected_assignees = (old_assignees + new_assignees) - (old_assignees & new_assignees)
+          invalidate_cache_counts(affected_assignees.compact, issuable)
         end
 
         after_update(issuable)