Pick context windows for inlines in a slightly smarter way

Summary:
Ref T10694. This mostly prevents us from having a degenerate case if someone leaves a 200-line inline.

• For one-line inlines, show 1 line of context above and below (3 lines total).
• For 3+ line inlines, show just the inline.
• For 7+ line inlines, show only the first part.

Test Plan: Made a bunch of weird long/short/different-sized comments, saw reasonble-appearing context in text and HTML mail output.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10694

Differential Revision: https://secure.phabricator.com/D15853