Explicit rebase --onto support

Even with #6 (closed) it's not currently possible to use rebase --onto automatically with git-assembler. --onto specifies the new base, which we need to know in addition to the upstream branch in order to construct the dependency graph correctly.

Parsing --onto explicitly would be an option, but I don't see this as a robust solution. Adding an onto command might be better. The syntax in order to be consistent with the argument order of our old rebase (destination as 3rd) would look like this:

onto branch newbase oldbase -> "git rebase --onto newbase oldbase branch"

Overloading rebase itself might also be an option:

rebase branch oldbase -> "git rebase oldbase branch"

rebase branch oldbase newbase -> "git rebase --onto newbase oldbase branch"

which also kind of makes sense.

@erichdongubler @tiennou suggestions?