rust-update: handle SIGINT from long-running update threads The current code does not respond to ^C until after the Rust bit is finished doing its work. This is expected, since Rust holds the GIL for the duration of the call and does not call `PyErr_CheckSignals`. Freeing the GIL to do our work does not really improve anything since the Rust threads are still going, and the only way of cancelling a thread is by making it cooperate. So we do the following: - remember the SIGINT handler in hg-cpython and reset it after the call into core (see inline comment in `update.rs` about this) - make all update threads watch for a global `AtomicBool` being `true`, and if so stop their work - reset the global bool and exit early (i.e. before writing the dirstate) - raise SIGINT from `hg-cpython` if update returns `InterruptReceived`