rbtools.commands.post had two classes for managing diff results:

SquashedDiff and DiffHistory.
SquashedDiff represents a single diff result for any range of commits,

without any commit history information.
DiffHistory represents commits across multiple commits, with a

cumulative diff spanning all changes.
These have been internal to the rbt post command, defined as

namedtuples (despite not used tuple functionality for these). With

some work planned for cleaning up the posting logic, it was time to

clean up these classes and move them out of this command.
Both of these now live in rbtools.clients.base.diffs. They contain

standard utility methods for building an instance from a diff result

from a SCMClient, and inherit from a base class that holds common

fields.
One API change here is that DiffHistory.cumulative_diff has been

renamed to DiffHistory.diff. This was done to simplify code that may

take either object and needs to extract the diff, and to let us leverage

as many common fields as possible.
These are still just used in rbt post, but RBTools 5 will begin to

make use of these in other modules.

Unit tests pass.
Successfully posted using both commit histories and squashed diffs.