Make `uv init` resilient against broken git (#12895)
Currently, `uv init` works without a `git` executable, and with a
working `git` executable, but not with a broken `git`, be it from GitHub
Action's Windows CI or from the shim we insert.
`uv init` calls git twice: Once `git rev-parse` to check whether a git
repo already exists, and then `git init` (if there is no git repository
yet and no `--vcs none`).
By separately handling the cases where git failed during `git rev-parse`
doesn't work vs. where the is no repository when checking for an
existing repo work tree, we can avoid calling `git init` for broken git
and erroring. We have to hardcode the expected git command outputs to be
able to check.
