Give the rebuild its own call limit

A rebuild summarizes the whole merged log, so it can need more calls
than a probe sample. The import of two chats estimates 647 calls, close
to the 660 calls of the probe, and a stop at the limit swaps nothing and
wastes the calls. The rebuild now stops after 1,500 calls, and the quota
stop still guards the account. The probe keeps its own limit of 660
calls.

Issue: b0b8bf9