builtin/rev-list: migrate missing_objects cleanup to oidmap_clear_with_free()

builtin/rev-list: migrate missing_objects cleanup to oidmap_clear_with_free()

As part of the conversion away from oidmap_clear(), switch the
missing_objects map to use oidmap_clear_with_free().

missing_objects stores struct missing_objects_map_entry instances,
which own an xstrdup()'d path string in addition to the container
struct itself. Previously, rev-list manually freed entry->path
before calling oidmap_clear(&missing_objects, true).

Introduce a dedicated free callback and pass it to
oidmap_clear_with_free(), consolidating entry teardown into a
single place and making cleanup semantics explicit.

Signed-off-by: Seyi Kuforiji <kuforiji98@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>