Allow Fact analysis of commits

Summary: For immutable objects, just use the ID as a cursor.

Test Plan:

• Analyzed commits from an empty cursor.
• Checked that cursor was good.
• Pulled some more commits.
• Analyzed commits again, verified it only hit the new ones.
• Verified the graph of "Count of CMIT" looked reasonable.

Reviewers: vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1866

Differential Revision: https://secure.phabricator.com/D3656