Support date-range commit graph queries, and multiple disjoint commits in Git

Summary: Ref T13546. Allow the commit graph to be queried by date range, and Git to be queried for multiple disjoint commits.

Test Plan: Ran "arc branches" and future code which searches for alternate commit ranges for revisions.

Maniphest Tasks: T13546

Differential Revision: https://secure.phabricator.com/D21379