Fix bugs in git-rebase wrt rebasing another branch than
the current HEAD, rebasing with a dirty working dir,
and rebasing a proper decendant of the target branch.

[jc: with a bit of hand-merging]
Signed-off-by: NLukas Sandström <lukass@etek.chalmers.se>
Signed-off-by: NJunio C Hamano <junkio@cox.net>