packfile: use hashmap_entry in delta_base_cache_entry

packfile: use hashmap_entry in delta_base_cache_entry

This hashmap_entry_init function is intended to take a
hashmap_entry struct pointer, not a hashmap struct pointer.

This was not noticed because hashmap_entry_init takes a "void *"
arg instead of "struct hashmap_entry *", and the hashmap struct
is larger and can be cast into a hashmap_entry struct without
data corruption.

This has the beneficial side effect of reducing the size of
a delta_base_cache_entry from 104 bytes to 72 bytes on 64-bit
systems.

Signed-off-by: Eric Wong <e@80x24.org>
Reviewed-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>