Show an example of deleting commits with git-rebase.

Show an example of deleting commits with git-rebase.

This particular use of git-rebase to remove a single commit or a
range of commits from the history of a branch recently came up on
the mailing list.  Documenting the example should help other users
arrive at the same solution on their own.

It also was not obvious to the newcomer that git-rebase is able to
accept any commit for --onto <newbase> and <upstream>.  We should
at least minimally document this, as much of the language in
git-rebase's manpage refers to 'branch' rather than 'committish'.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>