Support an "Ancestors Of: ..." constraint in commit queries

Summary:
Ref T13137. See PHI609. An install would like to filter audit requests on a particular branch, e.g. "master".

This is difficult in the general case because we can not apply this constraint efficiently under every conceivable data shape, but we can do a reasonable job in most practical cases.

See T13137#238822 for more detailed discussion on the approach here.

This is a bit rough, but should do the job for now.

Test Plan:

• Filtered commits by various branches, e.g. "master"; "lfs". Saw correct-seeming results.
• Stubbed out the "just list everything" path to hit the diffusion.internal.ancestors path, saw the same correct-seeming results.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13137

Differential Revision: https://secure.phabricator.com/D19431