git sequencer prototype

Hi,

here is the patchset for the git-sequencer prototype, documentation,
test suite and a first git-am and git-rebase-i migration.
Indeed, monster patches. ;)

I'm using sequencer-based git-am and git-rebase-i and also git-sequencer
itself for around 2-3 weeks now. So, for me, it is reality-proven, but
I'm curious about your opinions/suggestions in usage and source.

The migration patches are a little hard to code-review in the diff-form,
but feel free to apply, test, and then look at the code ;)

Regards,
  Stephan