Allow non-linearity

[feefladder]

This repo and especially git-linearize does not exactly work as I would want it to work if there are dangling/merged branches. However, this would be very possible in a linear history like so:

non-linearized:

1a->2b->3c->4d->5e
         \      
         3b->3c

linearized:

1->2->3->4->5

1a->2b->3c->4d->5e
         \
          3->4

The main thing is that the branch does not come from the linearized git history