Use committer date instead of author date for Git epoch

Summary:
This is somewhat controversial but push date is usually more useful than commit date (which can be for example a month before other people can see the commit).
We can also store both dates.

Test Plan:

git log --pretty="%ct %at"

Reviewers: epriestley

Reviewed By: epriestley

CC: nh, aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2319