This provides a single implementation of get_change(). It makes use of

the new get_log(), and also a new diff() function in the backends. This

helps keep the code maintainable and bug-free.
I've removed the commit cache stuff that was part of get_change(), since

we never actually ever saved anything in the cache. It's probably

unlikely we'll need to fetch the same commit info very often, so instead

of fixing the caching, I've just opted to save some memory and accesses

and remove it.
There's a couple fixes in here I ran into for revision normalization as

well, and the removal of definitions and properties that are no longer

needed.

Unit tests pass for both backends.