Improve experience viewing large diffs.

[dmitshur]

Some commits can include generated files that are quite large (several MB). Current diff page will take a while to load, leading to a bad user experience. Especially when just trying to get a quick look (to see the commit message, files changed, etc.).

One step to improve this would be to avoid loading such files (either generated, or large) right away, but give the option to expand it later. That way the entire page loads quickly, and if a user really wants to see a large file, they can do so after explicitly clicking it.

For example, see second link in the commit message of commit 2536fa2.