from both files (hence two `-` removals from both file1 and
file2, plus `++` to mean one line that was added does not appear
in either file1 nor file2). Also eight other lines are the same
-from file1 but do not appear in file2 (hence prefixed with `{plus}`).
+from file1 but do not appear in file2 (hence prefixed with `+`).
When shown by `git diff-tree -c`, it compares the parents of a
merge commit with the merge result (i.e. file1..fileN are the