Add --continue and --abort options to git-rebase.

Add --continue and --abort options to git-rebase.

  git rebase [--onto <newbase>] <upstream> [<branch>]
  git rebase --continue
  git rebase --abort

Add "--continue" to restart the rebase process after
manually resolving conflicts.  The user is warned if
there are still differences between the index and the
working files.

Add "--abort" to restore the original branch, and
remove the .dotest working files.

Some minor additions to the git-rebase documentation.

[jc: fix that applies to the maintenance track has been dealt
 with separately.]

Signed-off-by: Junio C Hamano <junkio@cox.net>