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.

Unit tests pass.
Tested with an interdiff from the wild that resulted in lots of

exposed indentation changes from a merge. The indentation was no longer

shown.