Display context in changeset gaps

Summary:
I quite often wonder what's inside these gaps displayed in changeset. In which method I am? Is it a while loop or a foreach? Maybe I'm in a class but the project doesn't have a sctrict policy of one class per file so what's the name of the class?

I've experimented with bunch of rules:

• Always display 0 indentation: useless for one class per file.
• Always display 1 indentation: weird inside global functions.
• Display closest lower indentation: works best.

I'm not sure about highlighting the context. I like highlighting but maybe in this case subtler monochrome text will work better.

Test Plan: Browsed bunch of diffs, loved it.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3371