This change defines a TypedDict for diff chunks, and a tuple for
diff lines, with definitions and documentation for each element.
Before, these were just an opaque dictionary, and a heterogenous list.
The diff lines were particularly annoying because they were variable
length (if there was no metadata, that element was just left off the
end), and types were annoying (for "real" line numbers, we'd have an
int if the line was present on that side of the diff, but an empty
string if not). This change technically breaks compatibility in those
two cases.
We now always include the metadata field, set to None if there's no
metadata for the line. This increases the cached size of a very large
diff with thousands of lines by less that 0.1% (much better than many
years ago with older pickle protocols--the tuple is probably also
helping here).
In the case of real line numbers which are not present on one side of
the diff (for example, the line number of an added line for the
left-hand side of the diff), we set the data for that to None. In
order to maintain compatibility with current versions of Review Bot, we
change None line numbers back to an empty string for the filediff
resource's diff_data mode.
This change only touches code related to chunks and lines. Other diff
viewer cleanup is coming in later changes.
