packfile: define and use bsearch_pack()

packfile: define and use bsearch_pack()

The method bsearch_hash() generalizes binary searches using a
fanout table. The only consumer is currently find_pack_entry_one().
It requires a bit of pointer arithmetic to align the fanout table
and the lookup table depending on the pack-index version.

Extract the pack-index pointer arithmetic to a new method,
bsearch_pack(), so this can be re-used in other code paths.

Signed-off-by: Derrick Stolee <redacted>
Signed-off-by: Junio C Hamano <redacted>