Add a formatting atom, used with the --batch-check/--batch-command options,
that prints the octal representation of the object mode if a given revision
includes that information, e.g. one that follows the format
<tree-ish>:<path>. If the mode information does not exist, an empty string
is printed instead.
Signed-off-by: Johannes Schindelin <redacted> Signed-off-by: Victoria Dye <redacted> Signed-off-by: Junio C Hamano <redacted>