R1:5b640a434cc4

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…