test(git): silence bare repository fixtures

Two subprocess tests invoked git init through an inherited stdout and stderr without quiet mode. Clean Nix builds therefore printed Git's default-branch advice and repository initialization message twice in the test log.

Initialize the fixture explicitly on main and pass --quiet so it is independent of ambient branch configuration and emits no routine setup output. Production subprocess behavior is unchanged.

Validation: cargo fmt --all -- --check; cargo test -p ngit-grasp --lib git::subprocess::tests:: -- --nocapture.

Assisted-by: Codex (GPT-5)

src/git/subprocess.rs

@@ -142,7 +142,7 @@ mod tests {
     fn create_bare_repo() -> TempDir {
         let dir = TempDir::new().unwrap();
         let status = StdCommand::new("git")
-            .args(["init", "--bare"])
+            .args(["init", "--bare", "--quiet", "--initial-branch=main"])
             .arg(dir.path())
             .status()
             .expect("Failed to run git init");