Prerequisite

You have Emacs installed, the lastest version is Emacs 24.4. And Emacs instance run in server mode. Your Emacs will run as daemon, there will be only one Emacs window, you will open file with emacsclient.exe, the new buffer will be created in the daemon Emacs process.

You need to add this line to Emacs configuration script

 
(server-start)
 

You have Git installed on your computer.

Configure Git

Open command line window and run the following command

 
git config core.editor '/d/bin/emacs-24.4/bin/emacsclient.exe'
 

In case your Emacs is installed in d:\bin\emacs-24.4.

If this is now work, try this

 
git config core.editor '"d:/bin/emacs-24.4/bin/emacsclient.exe"'
 

When you commit changes from Git, the Emacs buffer will look like this:

 commit git message from emacs

After editing the commit message, exit the buffer, Emacs will ask you that there are clients exist, kill it anyway? Input yes and the commit should be success.