Improve rendering of commit branching graph

Summary:
Fixes T9323. Two minor fixes:

• On the first commit, don't render a downward line.
• Clean up a 1px spacing issue that had cropped up a while ago when we added icons or something, I think.

Test Plan:
Before:

{F1057248}

After:

{F1057249}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9323

Differential Revision: https://secure.phabricator.com/D14974