Parse and display commit authorship date in Git in Diffusion

Summary: Fixes T8826. Git tracks an "author date", which may be different from the "committed date". We don't currently extract/show this; do so.

Test Plan: {F1059235}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8826

Differential Revision: https://secure.phabricator.com/D14995