Create asset contains Git commit id that was commited at Build Unity project

_license

NYSL Version 0.9982 (en) (Unofficial)
----------------------------------------
A. This software is "Everyone'sWare". It means:
  Anybody who has this software can use it as if he/she is
  the author.
  A-1. Freeware. No fee is required.
  A-2. You can freely redistribute this software.
  A-3. You can freely modify this software. And the source
      may be used in any software with no limitation.
  A-4. When you release a modified version to public, you
      must publish it with your name.
B. The author is not responsible for any kind of damages or loss
  while using or misusing this software, which is distributed
  "AS IS". No warranty of any kind is expressed or implied.
  You use AT YOUR OWN RISK.
C. Copyrighted to Hassaku
D. Above three clauses are applied both to source and binary
  form of this software.

Commit.cs

using UnityEngine;

namespace Git {
    
    public class Commit : ScriptableObject{

        public string id;

        public static Commit Create(string commitId) {
            var instance = CreateInstance<Commit>();
            instance.id = commitId;
            return instance;
        }
    }
}

CommitIdText.cs

using UnityEngine;
using UnityEngine.UI;

//  sample of using commit id
[RequireComponent(typeof(Text))]
public class CommitIdText : MonoBehaviour{

    void Start() {
        Text text = GetComponent<Text>();
        Git.Commit commit = Resources.Load<Git.Commit>("GitHead");
        text.text = commit.id.Substring(0, 7);
    }
}

Preprocessor.cs

using System.IO;
using UnityEngine;
using UnityEditor;
using UnityEditor.Build;
using Git;

namespace Editor.Git {

    //  put in Editor folder
    //  requires Unity 5.6 or lator
    public class Preprocessor : IPreprocessBuild {

        public int callbackOrder {
            get { return 0; }
        }

        public void OnPreprocessBuild(BuildTarget target, string path) {
            var gitRoot = Path.Combine(Directory.GetCurrentDirectory(), ".git");
            if (!Directory.Exists(gitRoot)) {
                Debug.LogError("not fount .git directory");
                return;
            }

            var headPath = Path.Combine(gitRoot, "HEAD");
            var head = File.ReadAllText(headPath);

            string commitId;
            if (IsHeadDetached(head)) {
                commitId = head.Trim();
            }
            else {
                var headRefPath = Path.Combine(gitRoot, head.Remove(0, 4).Trim());
                commitId = File.ReadAllText(headRefPath);
            }
            Commit commit = Commit.Create(commitId);
            AssetDatabase.CreateAsset(commit, "Assets/Resources/GitHead.asset");  //  change asset path.
        }

        private static bool IsHeadDetached(string head) {
            return !head.StartsWith("ref:");
        }
    }

}