contents of <old|new>,
<old|new>-hex:: are the 40-hexdigit SHA1 hashes,
<old|new>-mode:: are the octal representation of the file modes.
-
+
The file parameters can point at the user's working file
(e.g. `new-file` in "git-diff-files"), `/dev/null` (e.g. `old-file`