R1:d952dd591266

When importing Git repositories, treat out-of-range timestamps as the current time

Summary:
Fixes T11537. See that task for discussion.

Although we could accommodate these faithfully, it requires a huge migration and affects one repository on one install which was written with buggy tools.

At least for now, just replace out-of-32-bit-range epoch values with the current time, which is often somewhat close to the real value.

Test Plan:
- Following the instructions in T11537, created commits in 40,000…