Perform "context-free" syntax highlighting for large files

[danvk]

If you try to diff two 20 MB JSON files with just a few changes, git diff will happily do it. But the webdiff UI might choke because it does syntax highlighting on both full 20MB files before displaying the diff hunks. It would be better if it didn't try to do this, and instead ignored the bits outside the diff hunk for syntax highlighting because JSON syntax highlighting isn't particularly sensitive to context.