PR performance problems

Lately, we've been seeing more and more PRs time out with the following error message:

"This merge is too large to display."

This doesn't seem necessarily correlated to merge sizes, for example, this change:

https://bitbucket.org/atlassian/amkt/pull-request/1249/amkt-6663-change-uid-for-atlassian-vendor/diff

is tiny and exhibits the same problem. Other scenarios where we're seeing the problem:

• PRs with small changes across many files
• Additions of large files
• Actual large PRs (typically refactorings)

We'll use this ticket as an umbrella issue to collect more data.