Filter out indentation changes in unrelated areas in interdiffs.

Our new interdiff logic tried hard to hide changes to files that weren't
part of the provided diff, but the new indentation markup code showed
those changes anyway.

Now instead of the interdiff code turning things into "equal" chunks, it
turns them into "filtered-equal" chunks instead. Those are then
converted back into "equal" chunks in a post-processing step, after the
indentation code has run. This prevents that code from operating on any
chunks we don't want to display changes for.