This change is a somewhat more invasive refactor of the ClearCase

backend in RBTools.
To start with, we had several different definitions of what a

"changeset" was throughout the code. Some places used it to mean a list

of 2-tuples containing extended path pairs. Some places used it to mean

a list of 3-tuples containing a path and two versions. And finally, our

diff implementation used it to mean a list of ChangesetEntry objects.
I've reworked this to define a "changelist" as the list of extended path

pairs (aliased as _ChangedEntry), which is what gets returned by all

of the different methods for the various types of revision specs. This

then gets processed and turned into a "changeset" which is the list of

_ChangesetEntry objects. A couple of the revision types use one

additional type, _BranchedChangedEntry, which is the 3-tuple form.
We also had a number of methods that were unnecessarily implemented as

their own functions. These have for the most part been folded back into

the one place they got called from.
Most of the code that deals with changelists has been updated to yield

items rather than build new lists. In some cases we were rebuilding

lists up to 7 times.
This change also adds type hints to the rest of the implementation.

Ran unit tests.