lake hangs at "Computing build jobs" when a file in an upstream project is deleted

[kim-em]

See commit 3ad5449927085b63f4304a13d534105004783bdd from Mathlib for a repro.

I've reported this a few times on zulip, but we better have a tracking issue.

[kim-em]

I think this should be high priority, as for the user of the downstream repository there is no mechanism to debug the problem.

[kim-em]

Actually, in the linked case above I'm not sure that it was a deleted file: Mathlib is pointing to Batteries @ lean-pr-testing-6112 there, which was a mistake (it should have been nightly-testing). Regardless, lake really shouldn't hang!