A deleted/nonexistent cwd also makes Bun.spawn throw ENOENT; only claim
'git is not installed.' when the working directory actually exists, else
name the missing directory. Addresses the Codex P2 review on #6237.
The read-only git helpers spawned `git` directly and only inspected the
result exit code. When git is absent from PATH, Bun's spawn throws ENOENT
(uv_spawn 'git') at launch, which escaped as an unhandled rejection and
crashed the process (e.g. Windows without git, relying on WSL git).
Translate a missing-binary launch failure into a non-zero GitCommandResult
in the async git() wrapper and into a null degrade in the sync reftable ref
readers, matching the existing non-zero-exit handling. Mutating/checked
commands keep surfacing the clean "git is not installed." error.
Fixes#6169
can1357