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 (LSPs no longer working (macOS 26 Tahoe) #36754 + RemiKalbe@6e8f2d2)
• 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.