Show a warning when "git" is too old to support filesize limits

Summary: See https://discourse.phabricator-community.org/t/git-push-failed-with-filesize-limit-on/2635/2

Test Plan: {F6378519}

Reviewers: amckinley, avivey

Reviewed By: avivey

Differential Revision: https://secure.phabricator.com/D20431