Add content from Arnd's email to git HowTo

[Matt Waddel]

Review Arnd's email versus our git HowTo to see if we can improve existing pages:

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> Then, I rebase them to Nicolas' linaro-2.6.39 using quilt.

This is unfortunately not the best approach. You should never
rebase a patch series to go on top of another patch series
or git tree other than the mainline linux-2.6.

You still have a large number of unrelated patches in a linear
series, which means that you are not one bit closer to getting
them merged upstream, and that you will have to do the exact same
work the next time when a new kernel gets released.

A better approach is to create multiple topic branches on top of
the mainline 2.6.39 kernel and apply patches from one area
of the kernel to each of that branch. In order to get a single
kernel tree from the multiple branches and the Linaro-2.6.39
changes, you then do a merge of all those branches into one master
branch.

This makes it much easier to see what patches you have in a
certain area of the kernel, or post the respective branch
for review. At the next upstream kernel release, you can take
the branches and rebase them individually. If one of the
branches causes problems, you can also let someone else do
the rebase.

I've tried to explain this some time ago and did an exemplary
git tree at http://git.linaro.org/gitweb?p=people/arnd/imx.git;a=summary
Unfortunately, it seems that this was not picked up in the main
imx tree, so you will have to do the work again.

Please do ask me or the linaro-kernel team when you have questions
about how to do the split into multiple branches. Once you have
some experience using git-rebase, it will be a very natural thing
to do.