stax rebase - support rebase for cases when parent is a commit or we can't rebase specific node

Consider following situation:
```
o       origin/master
o       checkpoint
| o     tmazepa/a
| | o   tmazepa/b
| | | o tmazepa/c
x-┘-┘-┘ HEAD
```

it would be good to allow rebasing from this position so all child branches could be rebased in one go without doing separate rebase for each child