The existing interdiff filtering algorithm tracks a single pending

opcode to process the next iteration, and yields filtered-equal

opcodes as they're calculated.
In preparation for some additional work on the algorithm that will

require opcode backtracking, this change switches to a

DiffOpcodeBuffer, which provides peek, consume, and queue operations

for the opcodes. This replaces the old pending_opcode, simplifies the

opcode loop, and gives us some look-ahead features we'll be using.
filtered-equal opcodes are no longer yielded as they're calculated.

Instead, they're added to a list, which tracks the original tag for

processing (in the upcoming algorithm changes). When it's time to yield

an opcode, it will first flush the filtered opcodes, turning them into

filtered-equal as they're yielded, and once that's flushed the new

non-filtered opcode will be yielded.
At the moment, this doesn't change anything about the processing of

interdiffs. The upcoming changes will take advantage of this to

backtrack and buffer opcodes through some new heuristics.

Unit tests pass.