Teach rebase interactive more commands to do better preserve merges

Junio proposed to add the commands mark, merge and reset to rebase
interactive for a better support of rebase with preserve merges. The
current format can only cope with flat lineare lists of commits and not
with the non‐linear structure of branches with merges.

This patch series is not intended for inclusion. It's a RFC and meant for
gathering ideas.

Currently it misses:
· commit messages
· documentation of the new commands
· update of the documentation of preserve merges
· tests for the merge command

I've included the patch to change the behaviour of git-merge from Junio,
because I don't know how to use the second syntax of git merge and pass
the merge strategies -s to git merge. Without this patch, a new merge
message gets appended to the commit message of a rebased merge commit.
This patch should not be part of the final commit series.

Open questions:
· Is it possible to get a list of all commits iterating the parents from
  the last to the first and going to the child of a node before the
  neighbors?

· How to get a symbolic name (branch or tag) for a commit? Not something
  like git describe. I want to know if 0123 refers to the tip of a
  branch.

Jörg.