Remove unnecessary property
BranchedCore datatype for generator

This allows arbitrary non-conflicitng (if working) branches
to be generated. Added commented out shrink code, not sure
how best to shrink but it is a start.
First rebase properties

Added two rebase properties, first being that it succeeds on
non-conflicting branches, the second that on non-conflicting
branches, the files are identical for "rebase a b" and "rebase b a"

TODO: Both currently FAIL. Need to figure out why