hook: introduce hook_config_cache_entry for per-hook data

hook: introduce hook_config_cache_entry for per-hook data

Replace the bare `char *command` util pointer stored in each string_list
item with a heap-allocated `struct hook_config_cache_entry` that carries
that command string.

This is just a refactoring with no behavior changes, to give the cache
entry room to grow, so it can carry the additional hook metadata we'll
be adding in the following commits.

Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>