hook: replace hook_list_clear() -> string_list_clear_func()

hook: replace hook_list_clear() -> string_list_clear_func()

Replace the custom function with string_list_clear_func() which
is a more common pattern for clearing a string_list.

To be able to do this, rework hook_clear() into hook_free(), so
it can be passed to string_list_clear_func().

A slight complication is the need to keep a copy of the internal
cb data free() pointer, however I think it's worth it since the
API becomes cleaner, e.g. no more calls with NULL function args
like hook_list_clear(hooks, NULL).

In other words, the callers don't need to keep track of hook
internal state to determine when cleanup is necessary or not
(pass NULL) because each `struct hook` now owns its data_free
callback.

Suggested-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>