under cygwin, update Android source code via repo

* Install cygwin

* make sure git version is 1.5.4 or greater than.

* Install repo

* Repo Installation

“Repo is a tool that makes it easier to work with GIT in the context of Android. For more information about Repo, see Using Repo and GIT.

To install, initialize, and configure Repo, follow these steps:

1. Make sure you have a ~/bin directory in your home directory, and check to be sure that this bin directory is in your path:
$ cd ~
$ mkdir bin
$ echo $PATH

2. Download the repo script and make sure it is executable:
$ curl http://android.git.kernel.org/repo >~/bin/repo
$ chmod a+x ~/bin/repo