Lines in a rendered diff have a dictionary field for storing move

information, and we have a couple other fields for other interesting

state. What we don't have is a field for general metadata, and this will

be important soon.
It will also be important not to completely break anything consuming

this line information. So, the moved information field is being

repurposed for general metadata, preserving to and from as moved

to/from keys.
For the moment, this is mostly a documentation change within the chunk

generator. However, the diff_line template tag has also been updated

to prepare for some uses of this metadata field. This consists of

pulling out the field into a variable, and changing some processing to

avoid repeating logic (which will be important for the upcoming code

safety checks feature).

Unit tests passed.
Made use of this in the upcoming code safety checkers feature.