Getting last commit with changes in specified subfolder in git repository

GitInfo.cs

internal class GitInfo : VcsInfoRetriever
{
	public override VcsInfoData Execute(string localPath)
	{
		string relativePath;
		var repoPath = FindRepoRoot(localPath, ".git\\index", out relativePath);
		if (repoPath == null) return null;

		using (var repo = new Repository(repoPath))
		{
			Commit lastCommit = repo.Head.Tip;
			if (!string.IsNullOrEmpty(relativePath))
			{
				var splitPath = relativePath.Replace('\\', '/').Trim('/').Split(Path.DirectorySeparatorChar);
				var changeTreeId = GetTreeId(repo.Head.Tip, splitPath);
				var previousCommit = repo.Head.Commits
							.TakeWhile(commit => GetTreeId(commit, splitPath) == changeTreeId)
							.LastOrDefault();
				if (previousCommit != null)
					lastCommit = previousCommit;
			}
			if (lastCommit == null) return null;

			return new VcsInfoData
			       	{
			       		LastChangedDate = new DateTime(lastCommit.Author.When.Ticks),
			       		LastChangedRevision = lastCommit.Sha,
			       		RevisionIsSequentialNumber = false,
			       		VcsName = "git",
			       	};
		}
	}

	private static string FindRepoRoot(string startPath, string metaFile, out string relativePath)
	{
		var completeStartPath = Path.GetFullPath(startPath);
		relativePath = null;
		string testPath;
		do
		{
			testPath = startPath;
			string metaPath = Path.Combine(testPath, metaFile);
			if (File.Exists(metaPath))
			{
				relativePath = completeStartPath.Substring(testPath.Length);
				return testPath;
			}
			startPath = Path.GetFullPath(Path.Combine(startPath, ".."));
		} while (testPath != startPath);
		return null;
	}

	private static ObjectId GetTreeId(Commit commit, IEnumerable<string> relativePath)
	{
		var root = commit.Tree;
		foreach (var folder in relativePath)
		{
			if (string.IsNullOrEmpty(folder)) continue;

			var treeEntry = root[folder];
			if (treeEntry == null) return null;

			root = (Tree)treeEntry.Target;
		}
		return root.Id;
	}
}