When restarting Zed, I hit a bug where all Git operations were hung. I
realized that there was a hanging git process running `git cat-file
--batch-check=%(objectname)`. The process was waiting on stdin. This was
surprising, because [the
code](https://github.com/zed-industries/zed/blob/e2bbdb19b6da2ee157ca1d36100acde2134a1663/crates/git/src/repository.rs#L1665-L1709)
that spawns this process explicitly closes the pipe that is attached to
the process's stdin after writing a list of ref names.

Using Claude, I found that this could be caused by that pipe file
descriptor being cloned due to file descriptor inheritance when another
child process is `exec`'d while that stdin pipe is open. The fix is to
enhance our Darwin process spawning layer to set the close-on-exec flag
for the pipe file descriptors, so that they are not inherited by child
processes spawned using code paths that don't set
`POSIX_SPAWN_CLOEXEC_DEFAULT`.

Release Notes:

- Fixed a bug on macOS where Git operations could be blocked depending
on the timing of spawning child processes.