test: end printed diff "hunks" with an empty string in test-context.py So that the resulting diff is correct and does not include a spurious empty line between lines "diff --git a/foo b/foo" and "--- a/foo".

mdiff: let _unidiff yield hunks as (<range information>, <hunk lines>) Now _unidiff yields each hunk lines packed into a tuple with the "range information" `(s1, l1, s2, l2)` that is used to build the typical hunk header '@@ -s1,l1 +s2,l2 @@'. This will be used to make it possible to filter diff hunks based on this range information. The new "range information" is ignored in unidiff() (only caller of _unidiff) for now.

mdiff: turn the comment above _unidiff into a docstring