util: Always use posix_spawn on macOS even with pre_exec hooks (#49090)

Here's some backstory:
* on macOS, @cole-miller and I noticed that since roughly Oct 2025, due
to some changes to latest macOS Tahoe, for any spawned child process we
needed to reset Mach exception ports
(https://github.com/zed-industries/zed/issues/36754 +
https://github.com/RemiKalbe/zed/commit/6e8f2d2ebe5a93753625a3026aeb996de8cb436b)
* the changes in that PR achieve that via `pre_exec` hook on
`std::process::Command` which then abandons `posix_spawn` syscall for
`fork` + `execve` dance on macOS (we tracked it down in Rust's std
implementation)
* as it turns out, `fork` + `execve` is pretty expensive on macOS
(apparently way more so than on other OSes like Linux) and `fork` takes
a process-wide lock on the allocator which is bad
* however, since we wanna reset exception ports on the child, the only
official way supported by Rust's std is to use `pre_exec` hook
* posix_spawn on macOS exposes this tho via a macOS specific extension
to that syscall `posix_spawnattr_setexceptionports_np` but there is no
way to use that via any standard interfaces in `std::process::Command`
* thus, it seemed like a good idea to instead create our own custom
Command wrapper that on non-macOS hosts is a zero-cost wrapper of
`smol::process::Command`, while on macOS we reimplement the minimum to
achieve `smol::process::Command`  with `posix_spawn` under-the-hood

Notably, this changeset improves git-blame in very large repos
significantly.

Release Notes:

- Fixed performance spawning child processes on macOS by always forcing
`posix_spawn` no matter what.

---------

Co-authored-by: Cole Miller <cole@zed.dev>

crates/git/src/commit.rs

@@ -80,16 +80,15 @@ pub async fn get_messages(working_directory: &Path, shas: &[Oid]) -> Result<Hash
 
 async fn get_messages_impl(working_directory: &Path, shas: &[Oid]) -> Result<Vec<String>> {
     const MARKER: &str = "<MARKER>";
-    let mut cmd = util::command::new_smol_command("git");
-    cmd.current_dir(working_directory)
+    let output = util::command::new_command("git")
+        .current_dir(working_directory)
         .arg("show")
         .arg("-s")
         .arg(format!("--format=%B{}", MARKER))
-        .args(shas.iter().map(ToString::to_string));
-    let output = cmd
+        .args(shas.iter().map(ToString::to_string))
         .output()
         .await
-        .with_context(|| format!("starting git blame process: {:?}", cmd))?;
+        .context("starting git show process")?;
     anyhow::ensure!(
         output.status.success(),
         "'git show' failed with error {:?}",