rev-list: extend print-info to print missing object type

rev-list: extend print-info to print missing object type

Additional information about missing objects found in git-rev-list(1)
can be printed by specifying the `print-info` missing action for the
`--missing` option. Extend this action to also print missing object type
information inferred from its containing object. This token follows the
form `type=<type>` and specifies the expected object type of the missing
object.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Acked-by: Christian Couder <christian.couder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>