R1:f4b9efe2562b

Introduce ref cursors for repository parsing

Summary:
Ref T4327. I want to make change parsing testable; one thing which is blocking this is that the Git discovery process is still part of `PullLocal` daemon instead of being part of `DiscoveryEngine`. The unit test stuff which I want to use for change parsing relies on `DiscoveryEngine` to discover repositories during unit tests.

The major reason git discovery isn't part of `DiscoveryEngine` is that it relies on the messy "autoclose" logic, which we…