Flaky test on Windows: test_git::test_concurrent_read_write_commit

[emesterhazy]

This test seems flaky. @martinvonz confirmed him and others have been seeing this for a while, but he hasn't been able to repro it on a Windows machine.

Here's an example flake:

• https://github.com/martinvonz/jj/actions/runs/7932098526/job/21657765413?pr=3065 (failed)
• https://github.com/martinvonz/jj/actions/runs/7932098526/job/21658408475?pr=3065 (successful retry)

Here's an excerpt from the logs:

---- test_git::test_concurrent_read_write_commit stdout ----
thread '<unnamed>' panicked at lib\tests\test_git.rs:2931:45:
unexpected error: A lock could not be obtained for reference 

I did a code search on github for the string "A lock could not be obtained for reference" and only appears in GitOxide here:

• https://github.com/Byron/gitoxide/blob/ccb607dcfc76246eb6a6edbbf8c3453a19d811cf/gix-ref/src/store/file/transaction/prepare.rs#L466-L469

GitOxide might not be the issue since Martin mentioned there's a failing run from 6 months ago with the same issue (the logs are missing so I can't confirm though).

• https://github.com/martinvonz/jj/actions/runs/5733763130/job/15538949575

Edit: I realized I should probably post the full log before it's gc'd by github actions.

failures:

---- test_git::test_concurrent_read_write_commit stdout ----
thread '<unnamed>' panicked at lib\tests\test_git.rs:2931:45:
unexpected error: A lock could not be obtained for reference "refs/jj/keep/c5c6efd6ac240102e7f047234c3cade55eedd621"
stack backtrace:
   0: std::panicking::begin_panic_handler
             at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library\std\src\panicking.rs:645
   1: core::panicking::panic_fmt
             at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library\core\src\panicking.rs:72
   2: runner::test_git::test_concurrent_read_write_commit::{{closure}}::{{closure}}::{{closure}}
   3: core::iter::adapters::filter_map::filter_map_try_fold::{{closure}}
   4: core::iter::traits::iterator::Iterator::try_fold
   5: <core::iter::adapters::filter_map::FilterMap<I,F> as core::iter::adapters::SourceIter>::as_inner
   6: <I as alloc::vec::in_place_collect::SpecInPlaceCollect<T,I>>::collect_in_place
   7: alloc::vec::Vec<T,A>::is_empty
   8: <alloc::vec::Vec<T> as core::iter::traits::collect::FromIterator<T>>::from_iter
   9: core::iter::traits::iterator::Iterator::collect
  10: itertools::Itertools::collect_vec
  11: runner::test_git::test_concurrent_read_write_commit::{{closure}}::{{closure}}
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
thread 'test_git::test_concurrent_read_write_commit' panicked at lib\tests\test_git.rs:2884:5:
a scoped thread panicked
stack backtrace:
   0: std::panicking::begin_panic_handler
             at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library\std\src\panicking.rs:645
   1: core::panicking::panic_fmt
             at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library\core\src\panicking.rs:72
   2: std::thread::scoped::scope
   3: runner::test_git::test_concurrent_read_write_commit::{{closure}}
   4: runner::test_git::test_concurrent_read_write_commit::{{closure}}
   5: core::ops::function::FnOnce::call_once
   6: core::ops::function::FnOnce::call_once
             at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library\core\src\ops\function.rs:250
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.


failures:
    test_git::test_concurrent_read_write_commit

test result: FAILED. 298 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 7.43s

error: test failed, to rerun pass `-p jj-lib --test runner`
Error: Process completed with exit code 1.

[emesterhazy]

Possibly related to #1536.