R1:cec3f44b900e

Provide an alternate, more general "closeable" flag for commits

Summary:
Ref T4327. This provides a more general RefCursor-based way to identify closeable commits, and moves away from the messy `seenOnBranches` stuff. Basically:

- When a closeable ref (like the branch "master") is updated, query the VCS for all commits which are ancestors of the new ref, but not ancestors of the existing closeable heads we previously knew about. This is basically all the commits which have been merged or moved onto the…