When lines 12, 13, 14, etc all blame to the same change, only show it once

Summary:
Depends on D19312. Ref T13105. For readability, render only one link for each contiguous block of changes.

Also make the actual rendering logic a little more defensible.

Test Plan: Viewed some files with blame, saw one render per chunk instead of one per line.

Maniphest Tasks: T13105

Differential Revision: https://secure.phabricator.com/D19313