[RFD] make rebase abort to original branch, not rebased branch

When git rebase is aborted, it will return the HEAD to the rebased branch (which has not moved). If the current branch is "topic" and a run of "git rebase master" is aborted, "topic" is therefore checked out, which is both what I expect and what I want. When the two-argument version of git rebase is used, as in "git rebase master topic", and then aborted, "topic" will be checked out. This is not what I would expect, although I'm aware of it by now, and usually not what I want. I would expect to be returned to the branch/commit that was checked out when I started "git rebase master topic".

In most cases, this is just a small annoyance, since it's usually quick and easy to manually switch back to the original branch. However, I have run into at least two cases where it has been a bit more annoying:

 1. When on a detached HEAD and running "git rebase HEAD topic", if
    you abort the rebase, you will have to look up the old commit in
    the reflog.

 2. I once was rebasing many old branches, mostly to see which ones
    could be rebased without conflicts. The project I was working on
    was pretty big and a lot had changed since those old branches were
    created. If a "new" branch is currently checked out and "git
    rebase master old-branch" is run and causes conflicts, the old
    branch would be checked out. In my case, this would take 5-10
    seconds. Since Junio improved rebase in 0cb0664 (rebase [--onto O]
    A B: omit needless checkout, 2008-03-15), there will be no
    checkouts of any "old" commit in the successful case, so it's a
    shame that it has to happen in the unsuccessful case when the
    rebase is aborted.

Are there valid cases where the current behavior is bettter?

I suppose we can not just change the current behavior due to backward compatibility constraints. We could introduce a configuration variable, of we could perhaps change it in 1.8.0. Maybe the problem is not big enough to do either...

/Martin