fix(refs): write stdin data to git update-ref subprocess

Use spawn + write_all + wait_with_output instead of output() so that
the constructed ref update commands are actually written to the
subprocess stdin.

refs/update_refs.rs

@@ -26,7 +26,7 @@ impl GitBare {
             return Ok(UpdateReferencesResponse::default());
         }
 
-        let output = std::process::Command::new("git")
+        let mut child = std::process::Command::new("git")
             .args([
                 "--git-dir",
                 &self.bare_dir.to_string_lossy(),
@@ -37,12 +37,29 @@ impl GitBare {
             .stdin(std::process::Stdio::piped())
             .stdout(std::process::Stdio::piped())
             .stderr(std::process::Stdio::piped())
-            .output()
+            .spawn()
             .map_err(|e| crate::error::GitError::CommandFailed {
                 status_code: None,
                 stderr: e.to_string(),
             })?;
 
+        if let Some(ref mut stdin) = child.stdin {
+            use std::io::Write;
+            stdin.write_all(stdin_input.as_bytes()).map_err(|e| {
+                crate::error::GitError::CommandFailed {
+                    status_code: None,
+                    stderr: format!("failed to write stdin: {e}"),
+                }
+            })?;
+        }
+        drop(child.stdin.take());
+        let output = child.wait_with_output().map_err(|e| {
+            crate::error::GitError::CommandFailed {
+                status_code: None,
+                stderr: e.to_string(),
+            }
+        })?;
+
         let stderr = String::from_utf8_lossy(&output.stderr).into_owned();
         if !output.status.success() {
             return Ok(UpdateReferencesResponse {