Google Summer of Code Guidelines
This year, Mercurial is trying to emphasize the community involvement aspect of GSoC, so the community bonding period is especially important. As a student, you should start by introducing yourself on the MailingLists and creating a WikiHomePage. You should also register on the BugTracker and start getting your feet wet by handling bugs and contributing patches. They don't have to be related to your GSoC project.
Much of the Mercurial development crew uses the MqExtension to develop patches, so you should start familiarizing yourself with mq as soon as possible. Your patch queue should be versioned and hosted online. Bitbucket is an excellent place to host your patch queue.
When you have patches to submit to mercurial-devel, please be sure to use the PatchbombExtension to send them. You can fire off patches from your queue directly to the list, and it will prevent any whitespace mangling issues from interfering with the review process.
Please keep communication public as much as possible. The MailingLists and the IRC channel (#mercurial on irc.freenode.net) are good places to solicit feedback.
Please go over the pages listed in DeveloperInfo. These documents will help you become versed in Mercurial's development practices.