Only cache MR diffs for one week

This may lead to some being evicted and having to be cached again, but many MRs
get closed or updated in that time anyway.

lib/gitlab/diff/file_collection/merge_request_diff.rb

@@ -72,7 +72,7 @@ module Gitlab
         end
 
         def store_highlight_cache
-          Rails.cache.write(cache_key, highlight_cache) if @highlight_cache_was_empty
+          Rails.cache.write(cache_key, highlight_cache, expires_in: 1.week) if @highlight_cache_was_empty
         end
 
         def cacheable?(diff_file)

spec/services/merge_requests/merge_request_diff_cache_service_spec.rb

@@ -15,7 +15,7 @@ describe MergeRequests::MergeRequestDiffCacheService, :use_clean_rails_memory_st
       cache_key = new_diff.diffs.cache_key
 
       expect(Rails.cache).to receive(:read).with(cache_key).and_call_original
-      expect(Rails.cache).to receive(:write).with(cache_key, anything).and_call_original
+      expect(Rails.cache).to receive(:write).with(cache_key, anything, anything).and_call_original
 
       subject.execute(merge_request, new_diff)
     end
@@ -31,7 +31,7 @@ describe MergeRequests::MergeRequestDiffCacheService, :use_clean_rails_memory_st
 
       expect(Rails.cache).to receive(:delete).with(old_cache_key).and_call_original
       expect(Rails.cache).to receive(:read).with(new_cache_key).and_call_original
-      expect(Rails.cache).to receive(:write).with(new_cache_key, anything).and_call_original
+      expect(Rails.cache).to receive(:write).with(new_cache_key, anything, anything).and_call_original
 
       subject.execute(merge_request, new_diff)
     end