Make block_with_timeout more robust (#11670)

The previous implementation relied on a background thread to wake up the
main thread,
which was prone to priority inversion under heavy load.

In a synthetic test, where we spawn 200 git processes while doing a 5ms
timeout, the old version blocked for 5-80ms, the new version blocks for
5.1-5.4ms.

Release Notes:

- Improved responsiveness of the main thread under high system load