Move detection has become a lot more complicated since it was first

introduced, and over time has become a significant portion of

DiffOpcodeGenerator's code. With some upcoming changes, it's going to

grow in complexity and will need to store some state on the instance

during processing. Rather than complicate the opcode generator even

more, I've split out the code into a new class.
The new MoveDetector take a differ and opcodes and runs through them,

running the move detection algorithm. It can be used without the rest of

DiffOpcodeGenerator now, which is handy for unit testing and might be

useful down the road for other purposes. We may also be able to rework

this down the road to detect moves across files, which would require

separating the detector from the opcode generator. For now, it keep the

code a lot more self-contained and organized and will open the doors to

some of the restructuring work I have planned to further improve the

detection of moved ranges.
The new code is the same as the old, except it's gained docstrings and

some clarifications, and has reduced the amount of attribute lookups

needed while iterating through files. Algorithmically, nothing has

changed.
DiffOpcodeGenerator has also gained a new enable_move_detection

option, which gives callers control over whether to perform move

detection. I plan to introduce similar options for other parts of the

opcode generation process in future changes.

Unit tests pass.
Viewed diffs using move detection. The new diffs exactly matched the

old ones.