More fine-grained options for showing additional context

[danvk]

webdiff (via codediff.js) currently gives you a link to show all skipped lines. It's sometimes nice to have an option to show a bit less than this.

Reviewable:

The "expand" links add 10 lines from either direction. The "39 lines with no changes" link shows all the skipped lines. The "expand 47 lines" presumably goes up to the hunk header, which in this case is in another section.

GitHub:

The "Expand Up" and "Expand Down" links give you 20 lines more context in either direction.