Fixes #19008.

git bisect run (and git rebase --exec) injects GIT_DIR into the
process environment before calling the user's command. When that command
is uv, the variable leaks into every git subprocess uv spawns,
directing them at the bisected repo instead of uv's own cache.

fetch_with_cli and fetch_lfs already stripped these five env vars
individually. The other call sites — open, init, rev_parse,
to_short_id, both clone variants, reset, and submodule update —
did not.

Changes

Introduce a git_cmd() helper:

fn git_cmd() -> Result<ProcessBuilder> {
    let mut cmd = ProcessBuilder::new(GIT.as_ref()?);
    cmd.env_remove(EnvVars::GIT_DIR)
        .env_remove(EnvVars::GIT_WORK_TREE)
        .env_remove(EnvVars::GIT_INDEX_FILE)
        .env_remove(EnvVars::GIT_OBJECT_DIRECTORY)
        .env_remove(EnvVars::GIT_ALTERNATE_OBJECT_DIRECTORIES);
    Ok(cmd)
}

All call sites now go through git_cmd(). The per-call env_remove
chains in fetch_with_cli and fetch_lfs are removed since the helper
covers them.

Before / After

Before: GIT_DIR=/bisected-repo/.git uv lock (with a git dependency) fails — uv's internal git rev-parse, git clone, etc. hit the wrong repo.

After: same command succeeds — git_cmd() strips the variable before each subprocess is spawned.

Test

Added lock_sdist_git_with_git_dir_env (behind #[cfg(feature = "test-git")]) that runs uv lock with GIT_DIR=/nonexistent in the environment and expects success.