builtin/repack.c: rename "struct generated_pack_data"

builtin/repack.c: rename "struct generated_pack_data"

The name "generated_pack_data" is somewhat redundant, since the contents
of the struct *is* the data associated with the generated pack.

Rename the structure to just "generated_pack", resulting in less awkward
function names, like "generated_pack_has_ext()" which is preferable to
"generated_pack_data_has_ext()".

Rename a few related functions to align with the convention that
functions to do with a struct "S" should be prefixed with "S_".

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>