rebase: learn to rebase root commit

rebase: learn to rebase root commit

Teach git-rebase a new option --root, which instructs it to rebase the
entire history leading up to <branch>.  This option must be used with
--onto <newbase>, and causes commits that already exist in <newbase>
to be skipped.  (Normal operation skips commits that already exist in
<upstream> instead.)

One possible use-case is with git-svn: suppose you start hacking
(perhaps offline) on a new project, but later notice you want to
commit this work to SVN.  You will have to rebase the entire history,
including the root commit, on a (possibly empty) commit coming from
git-svn, to establish a history connection.  This previously had to
be done by cherry-picking the root commit manually.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>