True if Dict representeds the changes in Hash1 to FileOrHash2.
If Hash1 is unbound, it is unified with the previous of
FileOrHash2. Returns _{initial:true} if Hash1 is unbound and
FileOrHash2 is the initial commit. Dict contains:
from:Meta1
to:Meta2
Meta-data for the two diffed versions
data:UDiff
String holding unified diff representation of changes to the
data. Only present of data has changed