rev-parse: allow printing compatibility hash

rev-parse: allow printing compatibility hash

Right now, we have a way to print the storage hash, the input hash, and
the output hash, but we lack a way to print the compatibility hash.  Add
a new type to --show-object-format, compat, which prints this value.

If no compatibility hash exists, simply print a newline.  This is
important to allow users to use multiple options at once while still
getting unambiguous output.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>