object: clear grafts when clearing parsed object pool

object: clear grafts when clearing parsed object pool

We do not clear grafts part of the parsed object pool when clearing the
pool itself, which can lead to memory leaks when a repository is being
cleared.

Fix this by moving `reset_commit_grafts()` into "object.c" and making it
part of the `struct parsed_object_pool` interface such that we can call
it from `parsed_object_pool_clear()`. Adapt `parsed_object_pool_new()`
to take and store a reference to its owning repository, which is needed
by `unparse_commit()`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>