Reduce SQL timings in Projects::MergeRequests::DiffsController#show.json

The controller Projects::MergeRequests::DiffsController#show.json is one of our most frequently requested controllers with a 99th percentile (of SQL query timings) above 200 milliseconds. According to our daily overview the statistics are as follows:

• 33 000 requests per 12 hours
• 7.5% of the request time is spent in SQL

SQL timings per request:

• Mean: 94.64 ms
• 95th percentile: 159.39 ms
• 99th percentile: 709.33 ms

This controller should be improved so that the 99th percentile is consistently below 200 milliseconds, with an ideal target of 100 - 150 milliseconds.