Fix Diffusion line number links in Git

Summary:
When you click a line number link in Git from a branch tip, it takes you to
"...;origin/master$..." which (a) doesn't work and (b) doesn't permanently
reference the line.

Link to the "stable commit name" instead.

Also fix a few other bugs/warnings/layout things.

Test Plan:
Clicked line number links in Git and SVN repositories, browsed around stuff,
checked error log.

Reviewed By: jungejason
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran, epriestley, jungejason
Differential Revision: 303