Searches "%programfiles%/git/bin/" and "%programfiles (x86)%/git/bin/" for existing 'git.exe' (so you don't need git.exe in your environment path). Original source: http://versioning.codeplex.com/SourceControl/latest#MSBuildVersioning/GitInfoProvider.cs

GitInfoProvider.cs

using System;

namespace MSBuildVersioning
{
	/// <summary>
	/// Provides Mercurial information for a particular file path, by executing and scraping
	/// information from the hg.exe command-line program.
	/// </summary>
	public class GitInfoProvider : SourceControlInfoProvider
	{
		private int? revisionNumber;
		private string revisionId;
		private bool? isWorkingCopyDirty;
		private string branch;
		private string tags;

		public string GetTimeStamp(){
			return DateTime.Now.ToString("yyyyMMddHHmm");
		}
		
		public override string SourceControlName
		{
			get { return "Git"; }
		}

		public virtual int GetRevisionNumber()
		{
			if (revisionNumber == null)
			{
				InitRevision();
			}
			return (int)revisionNumber;
		}

		public virtual string GetRevisionId()
		{
			if (revisionId == null)
			{
				InitRevision();
			}
			return revisionId;
		}

		public object GetLongRevisionId()
		{
			throw new NotImplementedException();
		}

		public string GitCommand { get;set; }
		void UpdateCommand()
		{
			string GitCmd = "git.exe";
			string ProgramsX64 = System.Environment.GetFolderPath(System.Environment.SpecialFolder.ProgramFiles);
			string ProgramsX86 = System.Environment.GetFolderPath(System.Environment.SpecialFolder.ProgramFilesX86);
			
			string git86 = System.IO.Path.Combine(ProgramsX86,"git/bin/git.exe");
			string git64 = System.IO.Path.Combine(ProgramsX64,"git/bin/git.exe");
			
			if (System.IO.File.Exists(git86)) GitCmd = git86;
			if (System.IO.File.Exists(git64)) GitCmd = git64;
			
			GitCommand = GitCmd;
		}
		
		private void InitRevision()
		{
			UpdateCommand();
			
			ExecuteCommand(GitCommand, "rev-list HEAD", output =>
			               {
			               	if (revisionId == null)
			               	{
			               		revisionId = output;
			               		revisionNumber = 1;
			               	}
			               	else
			               	{
			               		revisionNumber += 1;
			               	}
			               },
			               null);
		}

		public virtual bool IsWorkingCopyDirty()
		{
			if (isWorkingCopyDirty == null)
			{
				UpdateCommand();
				ExecuteCommand(GitCommand, "diff-index --quiet HEAD", (exitCode, error) =>
				               {
				               	if (exitCode == 0)
				               	{
				               		isWorkingCopyDirty = false;
				               		return false;
				               	}
				               	else if (exitCode == 1)
				               	{
				               		isWorkingCopyDirty = true;
				               		return false;
				               	}
				               	else
				               	{
				               		return true;
				               	}
				               });
			}
			return (bool)isWorkingCopyDirty;
		}

		public virtual string GetBranch()
		{
			if (branch == null)
			{
				UpdateCommand();
				branch = ExecuteCommand(GitCommand, "describe --all")[0];
			}
			return branch;
		}

		public virtual string GetTags()
		{
			UpdateCommand();
			if (tags == null)
			{
				tags = ExecuteCommand(GitCommand, "describe")[0];
			}
			return tags;
		}
	}
}

GitVersionTokenReplacer.cs

using System;
using System.Collections.Generic;
using System.Text;
using System.Text.RegularExpressions;

namespace MSBuildVersioning
{
    /// <summary>
    /// Replaces tokens in a string with information from a <c>GitInfoProvider</c>.
    /// </summary>
    public class GitVersionTokenReplacer : VersionTokenReplacer
    {
        public GitVersionTokenReplacer(GitInfoProvider infoProvider)
        {
            AddToken("REVNUM", () => infoProvider.GetRevisionNumber().ToString());
            AddToken("REVNUM_MOD", x => (infoProvider.GetRevisionNumber() % x).ToString());
            AddToken("REVNUM_DIV", x => (infoProvider.GetRevisionNumber() / x).ToString());
            AddToken("REVID", () => infoProvider.GetRevisionId());
            AddToken("DIRTY", () => infoProvider.IsWorkingCopyDirty() ? "1" : "0");
            AddToken("BRANCH", () => infoProvider.GetBranch());
            AddToken("TAGS", () => infoProvider.GetTags());
            AddToken("TIMESTAMP", () => infoProvider.GetTimeStamp());
        }
    }
}