Given an index entry number, the idx_get() function returns an
(version agnostic) index_entry structure containing all of the
information required to unpack the corresponding object from
the '.pack' file.

Since the v1 and v2 file formats differ in the layout of the
object records, we provide two implementations of the get
function and initialise the function pointer appropriately.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>