codereview: add release branch support
codereview: add release branch support

Note that if you are working on the upcoming release
branch you have to point your extension path to a
copy of lib/codereview/codereview.py that won't change
as the repository flips between release-branch and default branch.
This warning should only apply to this one branch and only to rsc,
but you never know.

R=adg
CC=golang-dev
http://codereview.appspot.com/4446076

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