Add option to rebase onto project's manifest version

Some teams have a continuous build server that would mark certain
manifest green and safe to sync to.  Then team members could repo
sync to that particular manifest file and make sure they always
sync to a green build.  But if she/he has some local changes and
wants to rebase, currently it would be a manual process to find the
correct version to rebase onto.  This patch helps with that use
case by automating the process to rebase onto the currently synced
manifest version.

Change-Id: I847c9eb6addf7f84fd3f5594fbf8c0bcc103f9a5