Performance and timeouts when loading tree with thousands of files

Zendesk: https://gitlab.zendesk.com/agent/tickets/84200

Customer has a couple of repositories that have 25,000+ files in certain directories. When one of these directories is opened in the repository file viewer the request will time out (even when increased to 300 seconds). It appears we may be doing some caching because it seems like after trying many times to load some of these pages they will eventually load quite fast. It's important to realize that browsing through 25,000 files in a directory is not really that useful. All the same, GitLab should handle it gracefully and load X files and a message saying that Y are not displayed due to large number.

cc/ @jramsay