merge.mergestate: add a generator for driver-resolved files Just like for unresolved files above, we need to be able to tell what files are driver-resolved.

hook: for python hooks, also return whether an exception was raised The hook code treats python hooks raising an exception and returning True as the exact same. This is OK for hooks themselves, but other code that wants to invoke external code using the same underlying code is a bit more interested in making a distinction.