Tidy context displayed in gap

Summary:
We have some complaints on this feature:

• It's not clear that the displayed code is context from gap.
• It would be better if the context would be displayed with its real indentation.
• It's not clear how far the context is from the displayed code.
• Links revealing gap aren't on consistent place.

This solves all these problems and introduces new one:
It now seems that the reveal links works only with the left side.
Anyway, I think that this is better overall.

I don't want to put the context on a separate line to not waste space.

Test Plan: Displayed various contexts, revealed context.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, bh, jwatzman

Differential Revision: https://secure.phabricator.com/D3404