In his November 30, 2011 post, Jean-Baptiste Kerou explains why the source of the kernel is invisible after cloning. This post describes the kernel source code branches that exist for various ICS hardware accessories.
In general, here's how to make a visible branch visible:
git clone https://android.googlesource.com/kernel/common.git
git branch -a
git checkout -b android-3.0 remotes/origin/android-3.0
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