git sequencer prototype

Hi,

for those who are interested in git-sequencer: here's the latest prototype, that should be able to apply to master.

An outline of the differences to the last sequencer prototype patchset:
 - typofixes, minor bugfixes
 - simplifications (seen due to builtin-ification)
 - introduced --allow-dirty
   that is used by the git-am migration, to allow
   usage of git-am on dirty index
 - set ("keep") ORIG_HEAD; somehow experimental

In the last patchset I mentioned the issue, that the prototype is slow as hell. I know some bottlenecks, but I have not even tried to change that, because this is no issue for the builtin.

I paste the experiments that I did on my test machine some time ago:

git-am: Apply 100 (trivial) patches
        original: 5.1s
 prototype-based: 17s  (wtf!)
   builtin-based: 2.8s

git-rebase--interactive: Pick 100 (trivial) commits
        original: 4.8s
 prototype-based: 10.1s
   builtin-based: 1.7s

Those times don't have any methodic value, it's just to get an impression.
Nevertheless some information about that:
 - performance was only tested one or two times
 - /proc/cpuinfo says my machine is an AMD 64 X2 with 4013 BogoMIPS
 - /bin/sh is dash (if the propaganda is true, bash is even slower)
 - the changes of the patches are equivalent to those of the commits

Regards,
  Stephan

Stephan Beyer (5):
  Add git-sequencer shell prototype
  Add git-sequencer documentation
  Add git-sequencer test suite (t3350)
  Migrate git-am to use git-sequencer
  Migrate rebase-i to sequencer

 .gitignore                      |    1 +
 Documentation/git-sequencer.txt |  676 +++++++++++++
 Makefile                        |    1 +
 command-list.txt                |    1 +
 git-am.sh                       |  632 +++++--------
 git-rebase--interactive.sh      |  436 ++-------
 git-rebase.sh                   |    7 +-
 git-sequencer.sh                | 2042 +++++++++++++++++++++++++++++++++++++++
 t/t3350-sequencer.sh            |  838 ++++++++++++++++
 t/t3404-rebase-interactive.sh   |    8 +-
 t/t4150-am.sh                   |    4 +-
 11 files changed, 3889 insertions(+), 757 deletions(-)
 create mode 100644 Documentation/git-sequencer.txt
 create mode 100755 git-sequencer.sh
 create mode 100755 t/t3350-sequencer.sh