Firstly, what's a "commit bit"? If you don't know what it is, you probably don't need one :-) But, in any case, a "commit bit" is the colloquial way that #perl6 describes how you obtain access to make commits directly to the rakudo repository. (i.e., not via a fork and pull request, but directly to https://github.com/rakudo/rakudo)
Step one, if you haven't already, is to send a signed copy of the Contributor License Agreement (CLA) to the Perl Foundation. The CLA can be found at http://www.perlfoundation.org/contributor_license_agreement
Why is a signed CLA necessary for commit access? Historically, it is because Rakudo used to be part of the Parrot project and that project required a signed CLA. But the reasons that Parrot required a CLA are also valid for Rakudo. That is, the CLA helps protect the Perl Foundation and the Rakudo project and contributors alike from intellectual property issues. The CLA warrants that your contributions are yours to give and that they are not encumbered by anyone else's intellectual "lien" on those contributions.
For instance, some employment contracts stipulate that any intellectual property that you generate belongs to your employer whether you generate it during work or after hours at home or while on vacation. If your employment contract has such a stipulation, you must seek permission from your employer such that you can contribute to Rakudo without fear that your employer will later claim ownership of your contributions. The CLA says that you have done the due diligence in these matters and that as far as you are aware, your contributions are yours alone to give.
Once you send off your signed CLA, you can then inquire on irc://freenode.net/#perl6 about getting commit access to the Rakudo repository. The main source code repository for Rakudo is administered on GitHub, so it would be a good idea to have a github account before you request a commit bit. There is at least one officer of the Perl Foundation on #perl6 who can verify the reception of your signed CLA and several people that can then add you to the Rakudo project.