Project Skara was created "to ... investigate alternative SCM and code review options for the JDK source code, including options based upon Git rather than Mercurial, and including options hosted by third parties." The OpenJDK skara-dev mailing list included a post from Robin Westberg last week that announced, "We have added some additional read-only mirrors of a few different OpenJDK project repositories to the https://github.com/openjdk group..."
The read-only OpenJDK repositories on GitHub will likely be more convenient for developers wanting to take advantage of the "open source" nature of OpenJDK to take a peek at its internals. More developers are likely to be comfortable with Git than with Mercurial. The GitHub-hosted repositories make it even easier to clone a given repository or to even fork it.